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Yolk sac during pregnancy.
The yolk sac is formed simultaneously with the amnion from the extraembryonic endoderm in the form of a yolk vesicle, which gives rise to the epithelial lining, and an adjacent layer of extraembryonic mesoderm (mesenchyme), which forms the connective tissue basis of the organ. Yolk sac in humans, it does not contain a yolk, but is filled with a liquid containing proteins and salts. Unlike birds, it does not play a significant role in providing nutrition to the embryo (the nutrition of the human embryo comes from the mother’s blood). It acts as the first hematopoietic organ: from its mesenchyme the first blood vessels, and inside them are blood cells (intravascular hematopoiesis). Primary germ cells (gonoblasts) appear in the endoderm of the yolk sac, which then migrate to the primordia of the gonads. After the formation of the trunk fold, the yolk sac remains connected to the intestinal tube by a small yolk stalk. Starting from the 7th - 8th week of embryogenesis, reverse development of the yolk sac is observed. The remains of the latter can be found in the umbilical cord in the form of a narrow epithelial tube.
Allantois
The allantois is formed on the 16th day of embryogenesis in the form of a finger-shaped outgrowth of the intestinal endoderm, which, heading into the amniotic leg, is surrounded by extra-embryonic mesenchyme. In birds, it performs the function of a urinary sac (it accumulates fetal excretion products that cannot be excreted in environment because of the shell). In humans, it does not reach significant development and serves to conduct blood vessels from the embryo to the chorion. Their thinnest branches, together with the mesenchyme, penetrate into its villi. Through the vessels of the allantois on early stages During embryogenesis, the functions of nutrition, gas exchange and release of the embryo are carried out. Starting from the second month, the allantois is reduced and remains in the form of a cord of cells in the umbilical cord along with a reduced yolk sac.
Chorion
The chorion, or villous membrane, is present only in placental mammals and humans. It is formed in the 2nd week of human development, when the extraembryonic mesoderm grows towards the trophoblast, forming secondary villi together with it. At the beginning of the third week, blood vessels grow into the chorionic villi, and they are called tertiary villi. Further development of the chorion is associated with the formation of the placenta.
Placenta
The placenta is the organ of communication between the fetus and the maternal body. It performs all functions necessary for the development of the fetus. Respiratory, trophic, excretory functions are ensured due to the fact that oxygen and nutrients flow through the placenta from the mother’s blood into the blood of the fetus, and in the opposite direction - carbon dioxide and metabolic products.
The endocrine function is the production of hormones that regulate fetal development and the course of pregnancy: human chorionic gonadotropin, placental lactogen, progesterone, estrogens, melanotropin, corticotropin, somatostatin and others. The protective function of the placenta is to passively immunize the fetus with the help of maternal immunoglobulins. At the same time, the placenta provides an immune barrier between the mother’s body and the fetus’ body. Many micro- and macroelements, vitamins A, C, D, E, etc. are also deposited here.
The human placenta is formed from the 3rd to the 12th week of embryogenesis. It distinguishes between the fetal and maternal parts. The fetal part is represented by the chorionic plate, from which the villi extend. The maternal part includes a part of the lining of the uterus called the basal lamina.
The chorionic plate and villi of the fetal part of the placenta are formed by loose fibrous connective tissue covered with epithelium - cyto- and symplastotrophoblast. Some of the villi reach the basal lamina of the maternal part of the placenta and attach to it, forming anchor villi. The stem villi with their branches form the structural and functional units of the placenta - cotyledons, the total number of which reaches 200. Cotyledons are separated by septa extending from the basal plate of the maternal part of the placenta. The basal lamina is the deep layer of the endometrial lining that sheds. Here, in large quantities Decidual cells are found - large, glycogen-rich cells with oxyphilic cytoplasm and large nuclei. Their participation in the immune response and hormone production is assumed.
In humans, the placenta is discoidal in shape, and hemochorial in relation to the chorion to the maternal mucosa. The last name is due to the fact that the chorion receives oxygen and nutrients directly from the mother’s blood, located in the lacunae of the mother’s mucous membrane: “the chorionic villi are bathed in the mother’s blood.”
A hemochorial (placental) barrier is formed between the mother's blood and the fetus's blood. It consists of symplastotrophoblast, cytotrophoblast, epithelial basement membrane, villous connective tissue, fetal vascular basement membrane, fetal vascular endothelium. By the end of pregnancy, the trophoblast becomes very thin or disappears, being replaced by a fibrin-like oxyphilic mass, consisting of plasma coagulation products and trophoblast breakdown - fibrinoid. Thanks to the placental barrier, normally foreign substances and bacteria do not penetrate into the fetal blood. However, it is not an obstacle to viruses, alcohol, nicotine and a number of other substances that are dangerous to the fetus, especially in the first 3 months of pregnancy, since they are not metabolized, but accumulate in its tissues and organs, as a result of which they can cause disorders embryonic development and ugliness.
It is so established by Mother Nature that each organ performs its assigned function in the body. Gradually, with the development of science, humanity has studied every organ and its importance in our body. Only with the advent of ultrasound equipment did doctors have the opportunity to look into the secret world of the origin of life, but this only added new questions that needed answers. One of these mysteries was the then unknown organ, the yolk sac.
By order of the Ministry of Health of the Russian Federation, all pregnant women registered in antenatal clinics at the place of residence, are required to undergo three ultrasound screenings for different dates gestation:
- 10-14 weeks;
- 20-24 weeks;
- 30-34 weeks.
First ultrasonography carried out from 10 to 14 weeks. But for more accurate data, it is better to do an ultrasound at the end of the first trimester. During this period, it is easier to detect abnormalities in the development of the embryo, and in the case of serious defects, it is safer for the woman’s health to get rid of the abnormally developing fetus.
An ultrasound scan, which is performed before the first screening, is carried out only to establish pregnancy. And we are not able to detect any pathologies or abnormalities, because in a short period of time the dimensions ovum can't afford it.
But the doctor may prescribe an ultrasound examination if necessary. more than three once.
Examination with a device using ultrasonic waves is carried out in two ways: through the abdominal wall or through the vagina.
Ultrasound in the first trimester is assessed according to the following indicators:
- Coccyx-parietal size. This is the size of the embryo from the crown to the tailbone. Every doctor has a table of the relationship between embryonic length and gestational age. KTE depends entirely on the period.
- Heart rate. This criterion allows us to identify congenital pathologies of cardio-vascular system. The doctor also has a table of normative indications that can be used to determine early hypoxia and heart defects.
- Thickness of the collar space. This is the length of the area between the skin of the embryo and soft tissues cervical vertebrae. The indicator helps to identify terrible diseases such as. The nuchal translucency disappears after 14 weeks of conception.
- Position of the chorion. Doctors call the placenta in the first trimester chorion. This standard indicates in which part of the uterus the fetus has taken its place.
- Nose bone size. Like other criteria, the length of the nasal bone during screening will help identify abnormalities in the development of the baby. If ossification of the bridge of the nose is not detected or it is too small, then this indicates a chromosomal abnormality. If no other violations are found, then there is no reason to panic.
- Yolk sac. This indicator is of particular importance as it helps to detect undeveloped pregnancy. There is a certain thread between the yolk sac and the result of gestation.
In addition to studies using ultrasound equipment, in the period from 10 to 12 weeks they do. Blood sampling must be taken on the same day on which the ultrasound was performed. The analysis will reveal the likelihood of having a child with chromosomal abnormalities.
What is a yolk sac?
The yolk sac or gestational sac is a circular sac attached to the abdominal cavity of the embryo. Inside the sac there is a vital yolk that plays vital role in the development of the fertilized egg during placentation.
This organ is present in many mammals, birds, fish and cephalopods in the early stages of development and remains throughout life in the form of a cyst-shaped process in the intestine with remaining yolk.
Main functions of the yolk sac
Without this little bubble it is impossible full development fertilized egg. It takes on many functions, including nutrition and respiration of the embryo, while the appropriate organs for this are absent.
In addition to nutrition and respiration, the membrane membrane with the yolk serves as the primary circulatory system, through which oxygen and nutrients to the embryo.
Yolk sac during pregnancy
The gestational sac is a confirmation of a healthy intrauterine pregnancy. During ectopic gestation, this membranous membrane is not visualized. The “bag” appears in the second week of embryonic development and protects the fetus almost until the end of the first trimester, until other organs begin their work.
Between the fifth and sixth weeks the pouch should be clearly visible on ultrasonic ultrasound. This is one of the important criteria for the proper development of the embryo. The average diameter of the membrane shell is 5 mm.
Between the seventh and tenth weeks, the size of the bubble normally reaches up to 6 mm in diameter.
After 10 weeks, the yolk sac gradually ends its activity and must necessarily decrease in size. By the beginning of the second trimester, the fully formed placenta takes over the function of nutrition and breathing, and the yolk membrane is absorbed into the fetal cavity and in its place only a small appendage remains in the umbilical cord area.
In early pregnancy, an ultrasound is performed to identify a viable embryo in the uterine cavity, confirm the gestational age, exclude pathology of the embryo, or identify normal variants, such as multiple pregnancies.
The initial sign of pregnancy is thickening of the endometrium, but ultrasound does not tell what exactly causes this thickening.
When using a high-resolution transvaginal sensor, a gestational sac with a diameter of 1 mm is visualized in the uterine cavity 4 weeks and 2 days after last menstrual period with a regular menstrual cycle.
If menstruation is delayed for 5-7 days or more (gestational age is 5 weeks), a fertilized egg with a diameter of 6 mm should be clearly visible in the uterine cavity. It has a clear rounded shape with a fuzzy light rim along the periphery (hyperechoic rim - chorion). In this case, the level of beta-hCG in the blood is 1000-1500 IU/l (see What is hCG?). When the hCG level is more than 1500 IU/l, the fertilized egg in the uterine cavity should be clearly visualized.
With a lower level of hCG, the fertilized egg in the uterine cavity may not be detected by transvaginal echography. With a transabdominal examination, detection of the fertilized egg in the uterine cavity is possible at a beta-hCG level of 3000-5000 IU/l.
Fig.1Uterine pregnancy 4-5 weeks. Transabdominal scanning.
IMPORTANT: The gestational age cannot be accurately determined by the size of the fertilized egg. Many tables on the Internet with the size of the fertilized egg determine the period very approximately (see table below).
From about 5.5 weeks, transvaginal ultrasound begins to visualize an extraembryonic structure in the fetal egg - the yolk sac (eng. yolk sac). At the same time, the beta-hCG level averages approximately 7200 IU/l (see. hCG norms during pregnancy).
Since the yolk sac is part of the embryonic structures, its detection makes it possible to distinguish the fertilized egg from a simple accumulation of fluid in the uterine cavity between the endometrial layers, and in most cases, makes it possible to exclude an ectopic pregnancy. The incidence of ectopic pregnancy is 1-2 per 2000-3000 pregnancies. Its risk increases with the use of assisted reproductive technologies (ART). An ectopic pregnancy should be suspected when hCG level is more than 1500 IU/l, and the fertilized egg is not detected in the uterine cavity.
Fig.2Pregnancy 5.5 weeks. The yolk sac is identified. Transvaginal scan.
From 6 weeks of pregnancy (sometimes a little earlier), an embryo about 3 mm long can be identified in the fertilized egg. From the same date, the majority ultrasonic devices allows you to determine the heartbeat of the embryo. If the heartbeat is not detected or is unclear when the embryo length (CTE) is 5 mm, a repeat ultrasound scan is indicated after a week. The absence of cardiac activity at this stage is not necessarily a sign of fetal distress or a non-developing pregnancy.
The numerical values of the heart rate in an embryo during an uncomplicated pregnancy gradually increase from 110-130 beats/min at 6-8 weeks of pregnancy to 180 beats/min at 9-10 weeks.
The length of the embryo is measured from the head to the tail end, and is designated under the term CTP (coccygeal-parietal size), in English. literature - CRL (Crown-Rump Length). It should be noted that the coccygeal-parietal size of the embryo is less subject to individual fluctuations than the average internal diameter of the ovum, and therefore, its use to determine the gestational age gives better results. The error usually does not exceed ±3 days. With clear visualization of the embryo, the gestational age is determined depending on its length, and not on the size of the average internal diameter of the ovum (SVD).
To correctly measure the coccygeal-parietal size of the embryo, its clear visualization is necessary. In this case, one should strive to measure the maximum length of the embryo from its head end to the tailbone.
During normal pregnancy, the diameter of the ovum increases by 1 mm per day. Lower growth rates are a poor prognostic sign. With a gestation period of 6-7 weeks, the diameter of the ovum should be about 30 mm.
Table 1. Dependence of gestational age on the average internal diameter of the ovum (Dv), M. N. Skvortsova, M. V. Medvedev.
Table 2. Normal values of the coccygeal-parietal size (CPR) depending on the gestational age (full weeks + days), data in millimeters, the lower limit is the 5th percentile, the upper limit is the 95th percentile.
It should be emphasized that determining the gestational age by the length of the CTE is best done before 12 weeks of pregnancy. At a later date, measurements of biparietal diameter, head and abdominal circumference should be used.
Fig.3 Pregnancy 12 weeks 3 days.
The motor activity of the embryo is determined after 7 weeks of pregnancy. At first, these movements are very weak and isolated, barely noticeable during examination. Then, when differentiation into the head and pelvic ends of the embryo becomes possible, movements resemble flexion and extension of the body, then separate movements of the limbs appear. Since episodes of motor activity of the embryo are very short and count in seconds, and periods of motor rest can be significant in time, registration of the cardiac activity of the embryo is undoubtedly a more important criterion for assessing its vital activity.
The diagnosis of anembryonia (empty fertilized egg) is assumed if a yolk sac is not detected in a fertilized egg measuring 20 mm. Or if the fertilized egg with a diameter of more than 25 mm with a yolk sac does not contain an embryo. And also when the yolk sac size is 10 mm or more. In any case, if anembryonia is suspected, all data obtained should be interpreted in favor of pregnancy, and the study should be repeated after 7 days.
The diagnosis of a non-developing pregnancy should not be made if, on ultrasound, the ovum is less than 20 mm in size. When the embryo is 5 mm or more in length, in most cases the heartbeat should be clearly detectable. If the embryo is less than 5 mm, the ultrasound should be repeated after a week. If, upon repeated examination a week later, at CTE = 5-6 mm, cardiac activity is not detected, the pregnancy is not viable. The diagnosis of a non-developing pregnancy can be confirmed by a discrepancy between the beta-hCG level and echographic data.
It should be noted that the normal rate of pregnancy termination in the population is 15-20% of all clinically diagnosed pregnancies. However, in reality, if we count all “chemically” diagnosed pregnancies, determined by the level of beta-hCG before the expected next period, the miscarriage rate can reach up to 60%.
Best regards, doctor ultrasound diagnostics, Barto Ruslan Alexandrovich, 2012
All rights reserved®. Quoting only from written permission author of the article.
Ultrasound during pregnancy has long become an understandable and familiar procedure, because it is the simplest, most reliable and informative method monitoring the condition of the fetus. As a rule, a diagnostic specialist also determines the duration of pregnancy during the examination, and almost every patient takes this for granted, without thinking - how exactly does the doctor determine the timing? And how accurate? What parameters does it use for this?
It is diagnostics using ultrasound that helps doctors reliably determine the timing of gestation based on indicators of a very different nature, but at the same time directly established for a more informative result.
Setting an exact date also helps predict the date of birth, which is very important both from a medical point of view and simply to reassure the expectant mother. It is also necessary to know the most accurate period in order to control the development of the fetus, its condition, and the relationship between norms and reality.
How does a specialist determine deadlines?
Some patients believe that it is enough for the doctor to look at the image on the monitor - and the “age” of the child immediately becomes visually clear, but this, of course, is not the case. The specialist records the obtained data of the embryo and compares it with the normal variants. Usually, to help the doctor, there is a special table with fixed indicators, where all the relevant norms are listed by week.
Quantitative indicators.
Table No. 1. Norms of indicators at 5-10 weeks of pregnancy.
Table No. 2. Norms of indicators for the second trimester.
| 11 | 17-21 | 10-16 | 52-73 |
| 12 | 22-24 | 17-21 | 58-83 |
| 13 | 25-27 | 23-28 | 73-95 |
| 14 | 28-30 | 27-31 | 84-110 |
| 15 | 31-33 | 32-39 | 110 |
| 16 | 34-37 | 41-49 | 111-135 |
| 17 | 38-41 | 45-54 | 122-149 |
| 18 | 42-47 | 48-59 | 131-160 |
| 19 | 48-49 | 52-63 | 142-174 |
| 20 | 50-53 | 56-67 | 154-186 |
| 21 | 54-56 | 61-72 | 167-200 |
| 22 | 57-60 | 65-76 | 178-211 |
| 23 | 61-64 | 68-80 | 190-223 |
| 24 | 65-67 | 71-85 | 201-236 |
Table No. 3. Norms of indicators for the third trimester.
| Gestational age (in weeks) | BDP (biparietal size) (in millimeters) | Fronto-occipital size (in millimeters) | Head circumference (in millimeters) |
| 25 | 68-70 | 73-88 | 215-250 |
| 26 | 71-73 | 76-93 | 224-261 |
| 27 | 75-76 | 80-96 | 235-273 |
| 28 | 77-79 | 83-98 | 245-284 |
| 29 | 80-82 | 86-101 | 255-295 |
| 30 | 83-85 | 89-104 | 265-304 |
| 31 | 86-87 | 93-108 | 273-314 |
| 32 | 88-89 | 95-112 | 283-325 |
| 33 | 90-91 | 98-116 | 289-332 |
| 34 | 92-93 | 101-119 | 295-338 |
| 35 | 94-95 | 105-120 | 299-345 |
| 36 | 96-97 | 104-123 | 303-348 |
| 37 | 98-98 | 106-126 | 307-352 |
| 38 | 99-100 | 108-128 | 309-357 |
| 39 | 101-102 | 109-129 | 311-359 |
| 40 | 103 | 110-120 | 312-361 |
Table No. 4. Standards for fetal length.
| Gestational age (in weeks) | Embryo dimensions (in centimeters) |
| 5 | 0,8 |
| 6 | 1,1 |
| 7 | 1,3 |
| 8 | 1,5 |
| 9 | 2,2 |
| 10 | 3,2 |
| 11 | 4,1 |
| 12 | 5,3 |
| 13 | 7,5 |
| 14 | 8,7 |
| 15 | 10 |
| 16 | 11,5 |
| 17 | 13,1 |
| 18 | 14,2 |
| 19 | 15,2 |
| 20 | 16,5 |
| 21 | 26,6 |
| 22 | 27,8 |
| 23 | 29,8 |
| 24 | 31 |
| 25 | 34,6 |
| 26 | 35,5 |
| 27 | 36,5 |
| 28 | 37,7 |
| 29 | 38,6 |
| 30 | 39,8 |
| 31 | 41,1 |
| 32 | 42,5 |
| 33 | 43,6 |
| 34 | 45 |
| 35 | 46,1 |
| 36 | 47,3 |
| 37 | 48,6 |
| 38 | 49,8 |
| 39 | 50,6 |
| 40 | 51,7 |
| 41 | 52 |
| 42 | 53 |
Table No. 5. Standards for the abdominal circumference of the embryo.
| Gestational age (in weeks) | Abdominal circumference (in millimeters) |
| 11 | 40-61 |
| 12 | 50-71 |
| 13 | 58-79 |
| 14 | 66-91 |
| 15 | 91 |
| 16 | 88-115 |
| 17 | 93-130 |
| 18 | 105-144 |
| 19 | 114-154 |
| 20 | 125-163 |
| 21 | 137-177 |
| 22 | 148-190 |
| 23 | 160-201 |
| 24 | 173-223 |
| 25 | 183-228 |
| 26 | 194-240 |
| 27 | 206-253 |
| 28 | 217-264 |
| 29 | 228-277 |
| 30 | 238-290 |
| 31 | 247-300 |
| 32 | 258-314 |
| 33 | 267-334 |
| 34 | 276-336 |
| 35 | 285-344 |
| 36 | 292-353 |
| 37 | 300-360 |
| 38 | 304-368 |
| 39 | 310-375 |
| 40 | 313-380 |
Table No. 6. Norms for placental thickness.
| Gestational age (in weeks) | Optimal placental thickness (in millimeters) |
| 20 | 22-23 |
| 21 | 22,8-23,5 |
| 22 | 23,6-24,4 |
| 23 | 24,5-26 |
| 24 | 25,3-25,8 |
| 25 | 26,2-26,7 |
| 26 | 27-27,5 |
| 27 | 27,9-28,3 |
| 28 | 28,7-29 |
| 29 | 29,6-30 |
| 30 | 30,4-30,7 |
| 31 | 31,3-31,8 |
| 32 | 32,1-32,5 |
| 33 | 33-33,4 |
| 34 | 33,9-34,3 |
| 35 | 34,7-35 |
| 36 | 35,6-36 |
| 37 | 34,3-34,7 |
| 38 | 34,1-34,5 |
| 39 | 33,8-34 |
| 40 | 33,5-33,7 |
What exactly does the specialist analyze?
The analyzed norm indicators depend on the trimester and specific period.
In the first trimester Special attention is paid to the length of the fetus, since this is the only parameter that provides reliable information. During this period, there are no decisive differences in the development of embryos, so ultrasound establishes the “age” of the child accurate to the day.
The second and third trimester are periods during which absolute accuracy cannot be achieved, because from this moment the embryos begin to develop individually. Doctors use average statistical figures, but even in this case, the period is set as reliably as possible, and it is very possible to identify possible pathologies. At the same time, specialists analyze such indicators as the circumference of the child’s head, diameter chest, coccyx-parietal distance of the embryo.
Now it is necessary to analyze in more detail the meaning of the indicators presented in these tables, which may raise questions among patients.
Coccygeal-parietal distance - the distance, respectively, from the crown of the embryo to the coccyx. Thanks to this indicator, the most accurate determination of the period is possible, because these sizes are universal. Also, if there are factors that prevent the establishment of deadlines, KTR is the only reliable parameter in this case.
The diameter of the fertilized egg is the directly fertilized egg from which the embryo will develop in the future. Its dimensions, of course, directly depend on the specific period and have been studied for a long time - the doctor just needs to look at the corresponding table.
The diameter of the yolk sac is also an important indicator b, since the yolk sac plays a very important role throughout the development of the child (for example, in the first trimester it supplies the circulatory system of the embryo with nuclear red blood cells). The doctor always has the dimensions in the table.
Biparietal size– the distance between the parietal bones of the embryo.
Fronto-occipital size– the distance between the frontal and occipital bones, respectively.
Embryo length– is calculated when the child is in the most “unbent” state.
Placenta thickness– a lot depends on the placenta: it protects the child, supplies him with everything he needs, and produces many hormones. Therefore, it is very important to check that its thickness meets the specified deadlines. If the mother’s condition is stable, there are no deviations - it is enough to simply determine the “age” of the embryo by the thickness of the placenta.
Accuracy
Many women are interested in how accurately does fetal ultrasound help determine the timing by week? Therefore, it is important to note that ultrasound diagnostics establishes obstetric terms, in other words, the “age” of the embryo is calculated from the first day of the last menstruation. Sometimes a situation arises when a patient, checking tables on the Internet and ultrasound results, discovers differences in the period, so you need to know exactly what system was used to calculate, and not “sin” for the imperfections of ultrasound diagnostics.
Pathologies
Fetal ultrasound is informative not only as an aid in determining the due date, but also in detecting pathologies of various types. Moreover, some of them can only be detected using ultrasound, it is also important to note that since ultrasound can be done as often as desired, it is sufficient effective method monitoring the condition of the fetus.
So, the main pathologies that can be identified are:
- Developmental delay (identified by comparing normative parameters with reality; indicators below the threshold undoubtedly indicate the presence of an anomaly).
- Various defects (also detected when indicators do not match).
- Low water.
- Polyhydramnios.
- Thickening of the placenta.
- Non-developing pregnancy (indicators below standards (especially coccygeal-parietal size) in the first trimester).
When can I go to the diagnostic room?
Of course, modern ultrasound technologies make it possible to detect an embryo a week after conception, but most clinics still do not have such advanced equipment. It is also important to note that this will require a transvaginal ultrasound, which is very dangerous for the unborn child and may cause spontaneous miscarriage. The examination can be done so early only if there are special medical indications; in any other case, you can be patient with knowledge of the exact date.
It is recommended to contact a diagnostician at the 5th week of pregnancy, when it will be possible to examine the fetus in more detail using ultrasound, draw certain conclusions and maximum accuracy set a deadline.
Is it dangerous?
Some patients refuse ultrasound diagnostics using both the transvaginal and transabdominal methods, citing the dangers of ultrasound. Indeed, these fears are logical, because caring for the baby’s health is common to any mother. But ultrasound is not at all dangerous for the embryo; even at a short period of time, there is no reason to believe that ultrasonic waves will harm the development of the child, cause pathologies or cause a miscarriage.
Probability of error
Any woman can quite rightly consider that the size of the embryo is not the most reliable indicator for setting the due date, because each child develops differently and it is difficult to determine its exact age. But in fact, after years of medical practice, there is no longer any doubt about the correctness of the fetal due date established on ultrasound.
If a woman still questions the ultrasound results, she can always carry out a number of additional diagnostic procedures, which in turn will allow her to determine the timing at the most accurate level.
Fetal ultrasound is not only a method of monitoring the condition of the fetus, monitoring its development, but also a completely reliable, accurate, informative, convenient and simple way to determine the period by week. Based on many parameters, which have already been quite fully studied over the years of development of ultrasound medicine, the diagnostician quickly determines the obstetric “age” of the embryo, using special tables that a simple patient can rely on.
What is SVD during pregnancy and how to determine it on ultrasound? There is only one answer to this question.
SVD is the average internal diameter of the ovum according to ultrasound diagnostics. Measured this indicator exclusively in millimeters.
Fertilized egg illustration The gestation period is characterized by certain values of the internal diameter. The digital value of the SVD constantly varies, so the period is calculated with an error of a week to a week and a half. A more reliable sign is the CTE (coccygeal-parietal size) indicators. It should be noted that the coccygeal-parietal size of the embryo is less subject to individual fluctuations compared to the average internal diameter of the ovum, and therefore is used more often to establish a reliable gestation period. The error is approximately three days.
When the fetus is well visualized, the period is determined by the length of the fetus, and not by the internal diameter. The coccygeal-parietal size is recorded during a routine ultrasound and reflects the actual size of the fetus in combination with the approximate weight of the fetus. As a rule, the measurement of CTE indicators is used before, and in later ultrasound studies, the biparietal diameter of the circumference of the head and abdomen of the fetus is used.
Approximate indicators of SVD, depending on the timing of gestation
- When the diameter of the fertilized egg is approximately 4 millimeters, then the gestational age is. It is possible to assume that about four weeks have passed since conception.
- Closer to the fifth week, the diameter will reach 6 millimeters.
- After a few days, the fetus becomes 7 millimeters.
- the diameter increases to 12 - 18 millimeters.
- The average value of SVD at a period of six weeks and five days is 16 millimeters.
Fertilized egg on ultrasound
Of course, the expectant mother is concerned about the following question: how intensively does the fetus grow in the second and third trimester? We can say with confidence that its diameter is growing by one millimeter every day. Then its value increases by an average of 2 - 2.5 millimeters every day. During the border period of 16 - 17 weeks, they stop measuring the internal diameter of the ovum, focusing on more reliable indicators.
Ultrasound examination at short gestation
Diagnostics are carried out for the following purposes:
Diagnosis of the localization of the ovum 1. Establishing the exact location of the fetus (in the uterine cavity or outside it). When the fetus is located outside the uterus, we are talking about it. When the fetus cannot be visualized or the recognition process becomes significantly more difficult, they resort to precise definition embryo heartbeat. Signs of fetal viability may be found in the fallopian tubes or abdominal cavity.
Except this complication, in the initial stages of pregnancy, other complications may appear: for example, a changed shape of the fertilized egg; improper attachment; high risk placental abruption and other pathological disorders.
2. The definition of single or multiple pregnancy is not special labor. In the uterine cavity there are two or more fetuses with active life activity.
3. Assessing the main dimensions of the fertilized egg and embryo and comparing them with normal values.
4. Study of the correct structure of the embryo and fertilized egg to exclude serious congenital anomalies development. These may be chromosomal mutations (for example, Down syndrome).
5. Vital signs are assessed based on the presence of a heartbeat, which is detected already in the fifth week of gestation. The motor activity of the embryo is quite well determined after the seventh week of gestation.
On initial stage the movements are so weak and isolated that they can hardly be distinguished during an ultrasound. As the embryo grows, motor activity begins to resemble characteristic flexion and extension body movements, and then active movements of the upper and lower limbs. Since certain moments motor activity are quite short in time and counted in seconds or their fractions, then to register the fact of fetal life, the definition of cardiac activity is used.
6. . This small cystic formation provides the body expectant mother important hormones for preserving the fetus early stages development.
7. The study of amnion and chorion comes down to their ratio depending on the period of gestation already in the first trimester. Based on the obtained ultrasound results, it is possible to predict the further course and outcome of pregnancy.
Ultrasound is indispensable for determining possible problems with pregnancy
8. Diagnostics threatening miscarriage through ultrasound allows you to recognize early symptoms, which are characterized by a clear thickening of one of the walls of the uterine cavity, as well as a significant increase in internal pharynx. In case of a possible miscarriage, ultrasound assesses the vital signs of the fetus and the condition of the uterus and placenta as a whole.
9. Diagnosis of diseases and possible malformations of the female reproductive system (anomalies of the vagina or uterus). Any deviation from the norm determines the course and outcome of pregnancy.
Typical signs and features of fertilized egg implantation
Often, the fertilized egg attaches to the wall of the uterus several days after unprotected sexual intercourse, and then the egg is implanted into the endometrial layer after fertilization. WITH at this moment in the woman’s body the hormone hCG begins to be actively produced ( human chorionic gonadotropin), to which the pregnancy strip test reacts.
Implantation of fertilized egg
The screening test will not always be positive, so it is necessary to resort to a reliable blood test to determine hCG. After receiving positive result testing is necessary in as soon as possible contact a gynecologist at the antenatal clinic for registration and further monitoring for nine months.
The formed fertilized egg is the most sure sign the onset of pregnancy. It has a characteristic oval shape and is quite well visualized on ultrasound in the third week of absence of menstruation.
The embryo itself can only be seen when the period reaches the fifth week. If the ultrasound doctor does not detect an embryo in the fertilized egg, then the study is repeated after about half a month. Typically, the embryo becomes more clearly visible and its heartbeat is detected. In other cases we're talking about about pathological development or even about a frozen and undeveloped pregnancy.
That is why it is very important to undergo an ultrasound to exclude possible complications for further correction of the situation. The first trimester is the most important period of gestation, since throughout its entire duration all the organs and systems of the unborn baby are actively formed.
Timing of routine ultrasound diagnostics
Based on the WHO results, strict periods to conduct mandatory ultrasound examinations during the gestation period of the unborn baby.
Three ultrasound screenings are required
At other time intervals, the examination is prescribed strictly according to individual indications from the mother and fetus:
- recommended at 12 - 14 weeks;
- for 20 - 24 weeks;
- necessary at 32 - 34 weeks of gestation.
It is not advisable to neglect the timing of the next examination, since it is during the specified period of gestation that it is possible to recognize fetal malformations. And if a forced need arises, interruption by medical indications. The last screening examination can also be carried out at a later time.
The results of current diagnostics may be significantly outside the scope normal indicators, but this is far from a cause for concern. Do not forget that the development of each child has its own characteristics. However, you should not ignore the identified symptoms either.
) ultrasound examination is carried out to establish the localization (location) of the fetal egg. The fertilized sac is a round or ovoid (egg-shaped) formation that surrounds the embryo, usually located in the upper half of the uterine cavity. On an ultrasound, the fertilized egg looks like a small dark gray (almost black) spot with clear contours.
The presence of a fertilized egg in the uterine cavity eliminates the possibility ectopic pregnancy. In a multiple pregnancy, you can see two separately located fertilized eggs.
At what time can you see the fertilized egg?
Approximately two and a half weeks after conception, if menstruation is delayed by 3-5 days or more, that is, in the fourth to fifth obstetric week of pregnancy from the last day of the last menstruation, an ultrasound diagnostician can already see the fertilized egg in the uterine cavity using transvaginal ultrasound. The diagnostic level of hCG in the blood serum, at which the fertilized egg should be visible in the uterine cavity during transvaginal ultrasound, is from 1000 to 2000 IU.
The fertilized egg looks like a rounded black (anechoic or echo-negative, that is, not reflecting ultrasonic waves) formation, the diameter of which is very small and ranges from 2-3 mm. The embryo and extra-embryonic organs still have a microscopic structure and therefore are not yet visible using ultrasound. Using a parameter like average internal diameter of the ovum It is most advisable in the first 3-5 weeks of pregnancy from conception, when the embryo is not yet visible or is difficult to detect. The error when using the measurement usually does not exceed 6 days.
Fertilized egg: size by week
The size of the fertilized egg by week is very important indicator during pregnancy. For example, a gestational sac diameter of 3 mm corresponds to a gestational age of 4 weeks, and a gestational sac diameter of 6 mm corresponds to a gestational age of 5 weeks. An increase in the average diameter of the ovum occurs in the early stages of pregnancy at a rate of approximately 1 millimeter per day.
Most standard indicators for the average internal diameter of the ovum are limited to a period of 8-10 weeks. This is due to the fact that after 6–7 weeks of pregnancy, the size of the fertilized egg cannot reflect the growth of the embryo. With its advent, the coccygeal-parietal size of the embryo (CTE) is used to estimate the gestational age.
The dimensions of the average internal diameter of the ovum by week are given in the calculator.
Irregularly shaped ovum (deformed ovum)
If the fertilized egg is located in the uterine cavity, then such a pregnancy is called a physiological uterine pregnancy. A fertilized egg up to 5-6 weeks normally on ultrasound has a round or drop-shaped shape, surrounded by a thin membrane. By 6-7 weeks, it completely fills the uterine cavity and acquires an oval shape in a longitudinal scan, and a round shape in a transverse scan. If on an ultrasound the doctor sees a deformation of the fertilized egg (it is elongated, flattened on the sides, looks like a bean), then this may indicate uterine tone. Changing the shape of the fertilized egg is also possible with partial detachment. Significant deformation with unclear contours is observed during frozen pregnancy.
Timely diagnosis of deformation of the ovum during pregnancy makes it possible to save the child.
Empty fertilized egg
Normally, the fertilized egg in the uterine cavity is visible on transvaginal ultrasound approximately 32-36 days after the first day of the last menstruation. An important place is given yolk sac, which is of great importance in the development of the fertilized egg. At physiological course pregnancy, the yolk sac has a round shape, liquid contents, reaches maximum sizes by 7–8 weeks of pregnancy.
The embryo appears as a thickening along the edge of the yolk sac. The image of a normal embryo with a yolk sac looks like a "double bleb". By seven weeks, the yolk sac measures 4-5 mm. A relationship has been established between the size of the yolk sac and pregnancy outcome. When the diameter of the yolk sac is less than 2 mm and more than 5.6 mm, spontaneous miscarriage or non-developing pregnancy is quite often observed at 5–10 weeks.
The absence of a yolk sac with an average internal diameter of the ovum of at least 10 mm is an unfavorable ultrasound criterion for the threat of miscarriage.
An empty (false) ovum is an accumulation of fluid, usually irregular in shape, located near the border of the endometrium.
Sometimes there are cases when the fertilized egg has regular form and size, but there is no yolk sac or embryo inside it. The chorion of an empty fertilized egg produces hCG hormone, as with normal physiological pregnancy, so pregnancy tests will be positive. An ultrasound performed in the early stages of pregnancy can be erroneous, since the earlier it is done, the less chance there is of seeing the embryo. Before 7 weeks of pregnancy, a repeat study is required to clarify the diagnosis.
When on an ultrasound they see a fertilized egg in the uterine cavity, but do not see the embryo itself, doctors call this pathology anembryony (without embryo).
Non-developing pregnancy (embryo death) is indicated by following signs: altered membranes, absence of an embryo when the size of the ovum is more than 16 mm in diameter or the absence of a yolk sac when the membranes are more than 8 mm in size (when performing a transabdominal ultrasound: 25 mm - without an embryo and 20 mm - without a yolk sac); uneven contours, low location or absence of the double decidual sac.
In the early stages, the cause of pregnancy loss is most often chromosomal abnormalities that arose during the process of fertilization.
If a doctor finds a fertilized egg in the uterine cavity during an ultrasound, then you can congratulate the woman on her pregnancy. This formation in the uterine cavity is the very first and the most important sign development of pregnancy.
The formation contains the embryo as well as amniotic fluid. Depending on the shape, size and location of the structure, the doctor determines the nature of the pregnancy.
Having learned about their pregnancy, many curious expectant mothers begin to ask the doctor questions about how and at what stage the fertilized egg is visible and what it looks like. We will try to answer them.
The fertilized egg, the diameter of which is very small in the first days of pregnancy, can be seen within two to three weeks after a missed period. The formed structure in most cases is located in the upper part of the uterine cavity, has a dark (gray) tint and a round or oval shape. The embryo at this time is still microscopic in size, so it is not detected.
Development and structure
The growth of the fertilized egg begins from the moment of conception. The fertilized egg begins to move through the fallopian tube, during which cell fragmentation occurs. Making its way to the uterus, the fertilized, crushed egg needs nutrients and oxygen, so after a week a chorion begins to form on top, which subsequently transforms into.
The surface of the chorion has villi, which help the formation to attach to the uterus. In the future, these villi are contained only at the site of implantation of the formation into the uterine wall. The rest of the structure loses lint and remains smooth. The chorion provides the fetus with all vital important functions, one of which is protection against infections.
A value less than 7 mm indicates the middle of the fifth week. This is one of the most important periods when active formation blood vessels, heart and nervous system. The size of the embryo is usually 2 mm.
When an ultrasound shows a fertilized egg measuring 10 mm, this indicates that the heart and blood vessels are already fully formed and the embryo has a neural tube with a slight thickening at the end (the future brain).
6 obstetric week visualizes a value of 12 mm. At the 6th obstetric week, the fertilized egg is 12 mm in size, has a spherical shape, the embryo looks like white stripe about 5-6 mm long. At this point, the heart rate is 110-130 per minute. If any abnormality is detected during the sixth week, a repeat examination a week later is recommended.
To correct the situation, doctors remove and then accept the egg. correct form. What the fertilized egg looks like during a miscarriage depends on the gestation period. At 1-2 weeks, a miscarriage may look like menstrual bleeding. At later stages, the formation looks like a blood clot. If a miscarriage occurs at 7-9 weeks, the woman may find pieces of fetal tissue.
If the structure is oval and at the same time flat shape– this may also indicate. However, in the absence of pain and other ailments, it makes sense to continue monitoring the pregnancy. A repeated examination will allow the doctor to draw the correct conclusion.
Wrong location
A low gestational sac does not indicate a serious pathology, but requires more careful monitoring throughout pregnancy. If the formation is very close to the cervix, then it may occur cervical pregnancy, which is fraught with removal of the uterus.
Empty fertilized egg
When you can detect an empty fertilized egg, when the cavity contains only liquid or a blood clot.
Types of ultrasound. What are SVD and KTR?
To determine the parameters of the fertilized egg, different kinds Ultrasound:
- Transabdominal - examination occurs through the outer abdominal wall.
- Transvaginal – examination is carried out through the vagina.
During TA examination, clear identification of formation is possible starting from 5 obstetric week. At this time, the fertilized egg is 5-8 mm in size. Using the second research method, you can determine the size of the fertilized egg on the 3-6th day of missed menstruation, which is 4-5 weeks of gestation. The embryo is visualized starting from the 5th week of pregnancy during a TV examination, and during TA - from the 6th week in the form of a linear formation.
To assess the size and growth of the formation and embryo, the following indicators are used:
- SVD is the average internal diameter of the fertilized egg.
- KTP – coccygeal-parietal size of the embryo/fetus.
SVD shows the size of the fertilized egg by week and is measured in millimeters. Thus, the indicator of the size of the fertilized egg constantly varies over the weeks of pregnancy; the CTE indicator is more accurate for determining the reliable gestation period. At this study the error may be three days up or down. Basically, the study is carried out up to 12 weeks of gestation.
The size of the fertilized egg helps to quickly determine how far along the pregnancy is and how the fetus develops in the womb. The first three months of development are the most important, because it is during this time that all the organs and systems of the future baby are actively developing. Accordingly, it is important to undergo a scheduled ultrasound on time, which helps to identify possible deviations and carry out optimal correction of the current situation.
February 22nd, 2016 , 09:58 am
Yolk sac (Yolk sac).
The first extraembryonic structure visualized in the fertilized egg.
During embryonic development, this is the main route of exchange between mother and embryo.
- nutritious
- immunological
- metabolic
- endocrine
- hematopoietic
Accordingly, changes in the shape, size, and internal structure of the yolk sac may be associated with developmental prognosis for that particular pregnancy.
At ultrasound scanning becomes visible after the MSD (average internal diameter of the ovum) becomes greater than or equal to 8 mm (TV-ultrasound) - 5 - 5.5 weeks. menstrual age.
The yolk sac is a small ring-shaped formation located in the chorionic cavity with hyperechoic boundaries.
Using high-resolution TV-ultrasound, yolk sac can be detected at an MSD of 5 - 6 mm, but it is ALWAYS present when the ovum reaches a diameter of 8 mm.
On transabdominal scanning, the yolk sac should be visualized with an MSD of 20 mm or more.
Currently, the main function of detecting the yolk sac during ultrasound examination is to confirm intrauterine pregnancy.
At the 4th week of embryo development, the walls of the yolk sac consist of 3 thin cell layers. In the middle - mesodermal layer - already at 4 weeks a primitive capillary network is detected, in which hematopoietic stem cells are located. At the same age, hematopoietic cells are present inside the embryo. Initially near the developing heart. By the end of the 4th week, primitive blood cells are already widely scattered in the circulatory network of the embryo (in the rudiments of the heart, primary kidney and other embryonic organs).
The internal diameter of the yolk sac is from 3 to 5 mm.
A small yolk sac with a diameter of less than 3 mm between 6 - 10 weeks. or diameter greater than 7 mm before 9 weeks. pregnancy - suspicion of an abnormal pregnancy, which requires subsequent ultrasound monitoring to determine viability.
There is no clear consensus yet on what the upper limit of normal for the yolk sac should be: 5 or 6 mm at gestational age 5 - 10 weeks. Some studies show that the diameter of the yolk sac is more than 5 mm. Connected with increased risk spontaneous miscarriage. In any case, the excess is more than 5mm. requires more careful observation and subsequent ultrasound control.
Embryo (black arrow).
Chorionic cavity.
Vitelline duct (curved arrow).
Scheme B - transvaginal ultrasound of a living embryo 9 weeks 0 days.
A normal yolk sac (white arrow) within the gestational sac.
Embryo (black arrow).
Amniotic membrane (arrows).
Chorionic cavity.
Calipers are located on the inner edges of the yolk sac.
The size of the yolk sac gradually increases from the beginning of the 5th week to the end of the 10th week. Then it gradually decreases.
The yolk sac is connected to the embryo by the vitelline duct.
At 6 weeks of pregnancy, the yolk sac with the embryo adjacent to it (at this stage the size of the embryonic disc is 1 - 2 mm) looks like “ wedding ring with a diamond."
The embryo is ALWAYS present with a yolk sac with a diameter of 6 mm.
With TV ultrasound, the absence of a yolk sac in the presence of an embryo is always an anomaly and is associated with subsequent embryonic death.
A calcified yolk sac (echoic ring on ultrasound) is associated with embryonic death. Calcification occurs within a few days after the death of the embryo.
The clinical significance of an irregularly shaped yolk sac (irregular edges, areas of thickening) is currently under debate. However, an increasing number of studies are concluding that changes in the shape and size of the yolk sac may indicate or reflect significant dysfunction in this system.
Between 10 and 11 weeks, the yolk sac is rapidly reduced.
Options for yolk sac reduction:
- Parietal collapse.
- Cystic degeneration.
Early (before 10 weeks) reduction of the yolk sac is a marker of primary FPN.
Late (after 12 weeks, persistence) - marker intrauterine infection. With an ascending infection, the first barrier to the penetration of an infectious agent is the membranes. As a result of an infectious lesion, the process of fusion of the chorionic and amniotic cavities is delayed; in the unreduced chorionic cavity, infection of the yolk sac occurs, its size increases and its reverse development slows down.
When the vitelline duct is incompletely reduced, one of the most common developmental defects occurs in humans. gastrointestinal tract(2% of the population) - Meckel's diverticulum. A blind sac about 3 - 6 cm long in the distal ileum, which has its own blood supply. Its inflammation can mimic appendicitis.
It is so established by Mother Nature that each organ performs its assigned function in the body. Gradually, with the development of science, humanity has studied every organ and its importance in our body. Only with the advent of ultrasound equipment did doctors have the opportunity to look into the secret world of the origin of life, but this only added new questions that needed answers. One of these mysteries was the then unknown organ, the yolk sac.
By order of the Ministry of Health of the Russian Federation, all pregnant women registered with antenatal clinics at their place of residence are required to undergo ultrasound screening three times at different stages of gestation:
- 10-14 weeks;
- 20-24 weeks;
- 30-34 weeks.
The first ultrasound examination is carried out from 10 to 14 weeks. But for more accurate data, it is better to do an ultrasound at the end of the first trimester. During this period, it is easier to detect abnormalities in the development of the embryo, and in the case of serious defects, it is safer for the woman’s health to get rid of the abnormally developing fetus.
An ultrasound scan, which is performed before the first screening, is carried out only to establish pregnancy. And we are unable to detect any pathologies or abnormalities, because in a short period of time the size of the fertilized egg cannot allow this.
But the doctor may prescribe an ultrasound examination if necessary more than three times.
Examination with a device using ultrasonic waves is carried out in two ways: through the abdominal wall or through the vagina.
Ultrasound in the first trimester is assessed according to the following indicators:
- Coccyx-parietal size. This is the size of the embryo from the crown to the tailbone. Every doctor has a table of the relationship between embryonic length and gestational age. KTE depends entirely on the period.
- Heart rate. This criterion allows us to identify congenital pathologies of the cardiovascular system. The doctor also has a table of normative indications that can be used to determine early hypoxia and heart defects.
- Thickness of the collar space. This is the length of the area between the skin of the embryo and the soft tissues of the cervical vertebrae. The indicator helps to identify terrible diseases such as Down syndrome. The nuchal translucency disappears after 14 weeks of conception.
- Position of the chorion. Doctors call the placenta in the first trimester chorion. This standard indicates in which part of the uterus the fetus has taken its place.
- Nose bone size. Like other criteria, the length of the nasal bone during screening will help identify abnormalities in the development of the baby. If ossification of the bridge of the nose is not detected or it is too small, then this indicates a chromosomal abnormality. If no other violations are found, then there is no reason to panic.
- Yolk sac. This indicator is of particular importance, as it helps to detect an undeveloped pregnancy. There is a certain thread between the yolk sac and the result of gestation.
In addition to studies using ultrasound equipment, biochemical screening is done in the period from 10 to 12 weeks. Blood sampling must be taken on the same day on which the ultrasound was performed. The analysis will reveal the likelihood of having a child with chromosomal abnormalities.
What is a yolk sac?
The yolk sac or gestational sac is a circular sac attached to the abdominal cavity of the embryo. Inside the sac is the vital yolk, which plays a vital role in the development of the fertilized egg during placentation.
This organ is present in many mammals, birds, fish and cephalopods in the early stages of development and remains throughout life in the form of a cyst-shaped process in the intestine with remaining yolk.
Main functions of the yolk sac
Without this small bubble, the full development of the fertilized egg is impossible. It takes on many functions, including nutrition and respiration of the embryo, while the appropriate organs for this are absent.
In addition to nutrition and respiration, the membrane membrane with the yolk serves as the primary circulatory system, through which oxygen and nutrients are transferred to the embryo.
Yolk sac during pregnancy
The gestational sac is evidence of a healthy intrauterine pregnancy. During ectopic gestation, this membranous membrane is not visualized. The “bag” appears in the second week of embryonic development and protects the fetus almost until the end of the first trimester, until other organs begin their work.
Between the fifth and sixth weeks, the sac should be clearly visible on ultrasound. This is one of the important criteria for the proper development of the embryo. The average diameter of the membrane shell is 5 mm.
Between the seventh and tenth weeks, the size of the bubble normally reaches up to 6 mm in diameter.
After 10 weeks, the yolk sac gradually ends its activity and must necessarily decrease in size. By the beginning of the second trimester, the fully formed placenta takes over the function of nutrition and breathing, and the yolk membrane is absorbed into the fetal cavity and in its place only a small appendage remains in the umbilical cord area.
