Yolk sac 1 4 mm. At what age is the yolk sac visible?

This is how our consciousness works: for thousands of years, women have carried their children without thinking at all about some little things, for example, what stages of development the fetus goes through - the most important thing is that children are born viable and healthy. However, science has come a long way since then. Fetal ultrasound is now performed on every pregnant woman, and more than once. And just in the results of this examination, many discover that the fetus has some kind of yolk sac (sac, as many affectionately call it). And, of course, it becomes interesting where it came from, why it is needed and where it is found in an adult.

The yolk sac is formed at the beginning of the third week from the endoblastic vesicle (endoblast derivative). It does not function for long - about three months, but it has great value for fetal development. That is why the size of the yolk sac in the first six weeks after fertilization of the egg is much more sizes amniotic cavity along with the developing embryo.

Why is a yolk sac needed?

At the earliest stages of development, when the embryo is just forming, the yolk sac performs the most different functions, without which normal development is impossible.

WITH 18 -th day At the beginning of a new life, the first embryonic red blood cells (erythroblasts) are formed in the wall of the yolk sac and capillaries begin to grow, from which the entire circulatory system of the fetus is later formed.

WITH 28 -th day the walls of the yolk sac produce the first germ cells, which a little later will move to the gonads of the embryo. It is important to note that at this time the eggs of the future girl are laid. If at this stage the mother was ill or experienced severe stress– the laying of eggs will occur incorrectly in the future adult woman will suffer from infertility.

Before sixth weeks the yolk sac plays a role primary liver"and produces proteins very important for the development of the embryo, such as alpha-fetoprotein.

The yolk sac also takes an active part in metabolic processes, the formation of immunity and collects fetal secretions.

Where does the yolk sac go then?

At the end of the first trimester, the fetus is fully formed and switches to the placental type of blood circulation. The yolk sac becomes unnecessary and is reduced, that is, disappears. It remains as a small cyst at the base of the umbilical cord until the end of pregnancy. If the yolk sac is reduced ahead of schedule, when all the fetal organs have not yet formed, then the pregnancy will stop developing. The embryo simply will not be able to move to a new stage of development and become a fetus. This is what most often causes the development of frozen pregnancies and spontaneous abortions.

What about the ultrasound?

On ultrasound, the yolk sac is visible from the 6th to the 12th week from conception. If the doctor does not see the yolk sac, this is a very unfavorable sign: the pregnancy can stop developing at any time. Also unfavorable is the premature disappearance of the yolk sac.

Normally, the yolk sac should have the following dimensions:
less than 5.5 mm at 5–10 weeks;
more than 2 mm at 8–12 weeks.

What does it mean that the yolk sac is not visualized?

The yolk sac is formed 15-16 days after conception and ceases to function at the end of the first trimester - 11 weeks of pregnancy. The yolk sac plays an important role in the respiration and nutrition of the embryo, performs excretory and other functions before this function is taken over by the formed organs of the fetus and the placenta. After 12-13 weeks, the yolk sac is retracted into the fetal cavity, contracts and remains only as a cystic formation near the base of the umbilical cord. Yolk bag V normal visualized With 6 before 12 weeks pregnancy and is a round, thin-walled anechoic formation in the immediate vicinity of the embryo. Its diameter is 4-8 mm (at the 7th week from 4-5 mm), gradually increases to 10 mm, and then its reverse development begins.

If the yolk sac is not visible between 6 and 11 weeks:- Your gestational age may be incorrectly determined, and your pregnancy is less than 6 weeks (up to 6 weeks, the yolk sac is normally not visualized). In this case, ultrasound can already determine the heartbeat of the embryo (from 4-5 weeks) and the embryo is visible. - The absence of a yolk sac after 6 weeks may indicate an unfavorable course of pregnancy, which may end in termination or a frozen pregnancy. The prognosis is unfavorable. It is necessary to undergo an ultrasound examination again, preferably transvaginally with a high-resolution device, to verify the presence or absence of a yolk sac. - After 12 weeks it is normal if the yolk sac is not detected. This means that the fetus began to receive nutrients through the placenta.

The yolk sac is the earliest visible part of the fertilized egg. If it is visible on an ultrasound, this confirms the proper placement of the embryo. This is evidence that the egg has successfully implanted into the wall of the uterus, and the developing embryo has taken root in the womb, as it should be. So, the yolk sac: the norm by week.

Gestational sac and its role in embryo development

The yolk sac is a membrane membrane attached to the embryo on its ventral part. This formation provides early nutrition to the fetus. The gestational sac functions as the development of the cardiovascular system of the human embryo before internal circulation begins.

The gestational sac is the only available structure that can be used to determine the existence of an intrauterine pregnancy as long as the embryo cannot be identified. The yolk sac during pregnancy, if it develops normally, is visible on early stages using ultrasound. The gestational sac detected at the fifth week is a critical reference point in early pregnancy monitoring.

The normal functioning of the yolk sac is important at the beginning of the embryonic circulation. Transportation of blood to the walls of the sac occurs through the primitive aorta. Its circulation passes through a wide network of capillaries and returns through the vitelline vein to the tubular heart of the embryo. Through this circulation, nutrients from the yolk are absorbed and transferred to the embryo.

Read also:

At the beginning of the fifth week, the appearance of the yolk sac is a small pear-shaped umbilical vesicle opening into the digestive tract of the embryo in a long, narrow tube. It is called the vitelline duct. As a rule, the vitelline duct closes completely due to tissue proliferation during the 7th week. Subsequently, the child’s gastrointestinal tract is formed from it.

Ultrasound distinguishes the yolk sac as a small oval body, with a diameter ranging from 1-6 mm. When the development of the fetus inside the womb approaches the end of the 11th week, the functioning of the yolk sac stops, it decreases in size, reduces and appears as a cyst-like formation at the umbilical cord base. The membrane tissues that make up the fetal sac provide hematopoietic, excretory, immunoregulatory and synthetic functions, as well as metabolic processes, until the embryo develops its own organs and begins to function independently.

If the process of reduction of the fetal sac develops prematurely, before the liver, spleen, and reticuloendothelial system of the unborn baby are formed, then a miscarriage or fetal death may occur. Thus, three main stages of development of the yolk sac can be distinguished:

• The primary yolk sac is the umbilical vesicle formed during the second week of gestation.
• Secondary - by the end of the second week, a new cavity is released from the primary sac as a result of the division of hypoblast cells, the walls of which are transformed into two-layer ones. Meanwhile, the cells in the walls of the primary yolk sac degenerate.
• The final yolk sac forms during the fourth week of pregnancy during the development of the embryo's internal organs. Part of the yolk enters the digestive canal of the embryo. The remaining part is the final yolk sac.

Normal indicators by week

The yolk sac begins to form during the second week of embryonic development. Its full visualization on ultrasound occurs at 6 weeks. During normal development, the diameter of the yolk sac varies from 1 to 6 mm. The norms for the size of the yolk sac by week are shown in the table.

Ultrasound at the 5th week of gestation reveals only a small black dot. The normal diameter of the yolk sac should not be less than 2 mm. At week 6, embryonic structures are recognized along with the yolk sac. It contains nutrients and communicates with the embryonic intestine. The first blood vessels and blood cells are also produced here.

During the 7th week, the yolk sac is clearly visible, and its duct gradually closes due to tissue proliferation. The formation of the internal organs of the gastrointestinal tract of the embryo begins. The production of blood cells, the formation of the liver and spleen, and the cardiovascular system begins from the 8th week of pregnancy, and the yolk sac gradually reduces. Starting from the 9th week, the sex of the child is determined.

At week 10, the embryo's face takes on human features. Cells containing hormones appear and can balance blood sugar levels. At 11 weeks, the internal organs of the embryo are mostly formed and have begun to function independently. The yolk sac stops working and continues to reduce to a cystic state.

The normal course of pregnancy makes it possible to recognize the gestational sac at the 6th week and observe it until it is completely reduced at the end of the 1st trimester. If there is no visualization of the yolk sac, this means:

• It is possible that the gestational age was incorrectly determined and at the time of the ultrasound it is less than six weeks.
• Its absence during examination in the 7th week means the prognosis is unfavorable: miscarriage or fetal death is possible. There is a need to re-examine using a transvaginal method with higher resolution.
• The absence of visualization at the end of the 12th week indicates normal development of the embryo and the formation of the placenta, which provides its nutrition.

The conception of a child and its subsequent development in the womb is a series of complex stages of formation, interconnected by an indivisible and integral chain of processes. In early pregnancy, the most important link in this chain is the yolk sac, also known as the yolk sac. It should not be confused with the corpus luteum of the ovary, because these are two completely different organs of the same system.

Physiology of processes

First, it is important to determine the difference between the yolk sac and the corpus luteum. Both organs appear for a short time, participate in various processes of pregnancy development and are an integral part of it.

Yolk sac

This is a temporary extraembryonic organ. It should appear 15–16 days after conception during the placentation period, through the formation of embryonic tissue around the yolk of a fertilized egg. It is located on the ventral side of the embryo between the amnion and chorion, and is a semicircular formation with thin walls. In the body of a pregnant woman, the period of active functioning of the sac is limited to the first trimester. Starting from the 6th week it is clearly visible on ultrasound.

At 6–10 weeks of pregnancy, the diameter of the sac should not exceed 5.5 mm. At 8–12 weeks it should not be more than 2 mm. Starting from 12–13 weeks, the bubble can no longer be identified using ultrasound.

Corpus luteum

This is a temporary organ of the ovary, which begins its formation after the release of a mature egg from the dominant follicle, that is, after ovulation. It develops from granulosa cells of the ovulated ovarian follicle throughout the luteal phase, under the influence of luteinizing hormone.

If conception has occurred, the corpus luteum actively develops under the influence of human chorionic gonadotropin (hCG). The corpus luteum functions for 10–12 weeks after fertilization and is the main source of the pregnancy hormone progesterone. It is he who is responsible for maintaining and preserving pregnancy in its first stages. Progesterone inhibits the development of new eggs, the growth of the lining layer of the endometrium and, accordingly, stops menstruation. The corpus luteum fulfills its intended purpose until the placenta is completely formed and can independently produce the necessary hormones.

Purpose of the yolk sac

The yolk sac plays an important role in the formation of the embryo, especially in the early stages of its development. It performs a number of fundamental functions, without which further successful pregnancy will be impossible:

1. Performs the function of the liver and synthesizes important proteins: alpha 2-microglobulin, alpha-fetoprotein, transferrins, etc.
2. Transports the first germ cells to the embryo, as a result of which its gender is decided.
3. Forms the fetal immunity, acting as a macrophage system (reticuloendothelial system).
4. It forms in its walls the first red blood cells and a network of tiny capillaries, from which the circulatory system will be formed, feeding the embryo with all the necessary microelements.

After the 12th week of pregnancy, the bladder stops functioning, gradually shrinks and decreases in size. On the body of the embryo it remains in the form of a cystic formation near the umbilical cord.

Possible pathologies

When performing an ultrasound, doctors first of all pay attention to the shape and size of the yolk sac. If the examination revealed an irregular shape and size of the pouch, this may indicate the presence of various pathologies and even genetic abnormalities.

Imaging disorders

Normally, the yolk sac is visualized from 6 to 13 weeks. There are rare cases when this temporary organ is not displayed on the ultrasound machine screen in due time or, conversely, is visualized when it should not. There may be several reasons for this:

• If the pregnancy is short (less than 6 weeks), the yolk sac may not appear on the ultrasound machine screen. Normally, this organ is not visible until 6 weeks, but the fertilized egg and its heartbeat (5th week) are already there. It is this fact that can confuse the doctor and the pregnant woman.
• If the bubble is not visualized from the 6th to the 12th week of pregnancy, then there is a possibility that the pregnancy will end with a miscarriage or fetal death. The ultrasound examination must be repeated to exclude a technical error from the previous ultrasound or to confirm the pathology.
• Visualization of the sac after 12 weeks is a bad sign. By this time, the yolk sac should begin to reduce, and within the 13th week it should completely disappear and give way to the already formed placenta. If this organ continues to exist and does not decrease, then there is a high probability of a threat and pregnancy failure.
• Premature reduction of the sac (before 12 weeks). This sign is unfavorable and also indicates problems in the course of pregnancy, which may result in failure.
• There is a yolk bladder, but no embryo. A similar situation occurs during a frozen pregnancy, when the embryo is not laid or dies in the early stages, and the fertilized egg remains empty. In medicine, this case is called anembryonia. During anembryony, the yolk sac may appear, but the embryo itself does not.

Size mismatch

This pathology consists of an increased or decreased size of the yolk sac. The problem is considered dangerous for pregnancy, but at the same time it is a subjective sign that should be monitored by a doctor. In such cases, a triple test is first performed to confirm or deny the presence of anomalies. Triple test - study of markers of developmental defects and genetic disorders of the fetus: human chorionic gonadotropin(hCG), free estriol (E3) and alpha-fetoprotein (AFP). Diagnostics is also prescribed for sluggish reduction of the yolk sac after 12 weeks. In case of serious pathologies and anomalies of embryonic development, extreme measures to solve the problem are prescribed - abortion.

The problem of reduced size is usually associated with a lack of the hormone progesterone. In this case, the pregnant woman is prescribed drugs containing progesterone, for example, Urozhestan, Duphaston, Crinon, etc.

To summarize, it is worth noting once again that if there is a pregnancy, then when performing an ultrasound immediately after the delay, the corpus luteum of the ovary will be clearly visible, and from the 6th week the yolk sac will begin to be visualized. Non-compliance with standards for initial stage pregnancy can cause serious complications in the development of the fetus, so every pregnant woman needs to closely monitor her health from the very beginning until the happy end of pregnancy.

When discussing normal development of pregnancy in the early stages, from an echographic point of view, it is advisable to divide the first trimester into three periods: from 4 to 6 weeks, from 7 to 8 weeks and from 9 to 11 weeks.

During the embryonic period All major organs are formed. In the fetal period they grow and complete their functional development. Such a distinction is somewhat arbitrary and is based on terminology accepted in embryology. These terms, particularly gestational age, differ from those used clinically. Embryologists usually count gestational age from the moment of conception, while in clinical practice it is customary to use menstrual age, which is counted from the first day of the last menstrual period, since, as a rule, its date is known. Although the time interval between the first day of the last menstrual period and the moment of conception is usually two weeks, it can vary up to 8 days.
All data presented in this article is described based on calculating the gestational age based on the date of the last menstruation.

So-called " mid-embryonic period of development» falls within 4 to 6 weeks from the start of the last menstruation. Anatomy of the embryo in this early period development is usually still inaccessible for research due to the insufficient resolution of most currently used diagnostic instruments. Differences in the timing of ovulation (up to 12 days) and implantation (up to 3 days) may influence the findings obtained during transvaginal examination at these early stages of pregnancy.

With transvaginal echography One of the first signs of intrauterine pregnancy is an anechoic round inclusion with a diameter of only a few millimeters, located in the uterine cavity against the background of a thickened hyperechoic endometrium. A gestational sac can be detected no earlier than 4 weeks and 3 days, but is most often detected by transvaginal examination after 5 weeks.

At 5 weeks of pregnancy inside the fertilized egg, surrounded along the periphery by a layer of hyperechoic chorion, two cavities are defined, located one inside the other, which represent the developing primary yolk sac and the cavity of the extraembryonic coelom (exocoelom). This picture is revealed only within 2-3 days. An embryo that is not visualized by echography at this stage is designated as three-layered, since upon microscopic examination three distinct cellular layers can be distinguished (endoderm, mesenchyme and ectoderm).

Since transvaginal echography is relatively new clinical method, to establish reliable criteria for the relationship between the size of the ovum and visualization of the yolk sac and embryo, the need to accumulate additional clinical data remains relevant. Using the results obtained from examining patients who were treated with in vitro fertilization (IVF), it was found that the fertilized egg is usually visualized between the 4th and 5th week after the last menstrual period. Our experience shows that it can be detected after 4 weeks and 2 days, counting from the first day of the last menstruation. In most cases, with transvaginal echography, the yolk sac is visualized when the size of the fertilized egg reaches approximately 1 cm, and the yolk sac together with the embryo - when its size is about 1.5 cm.

Based on preliminary data it was found that during the period of detection of the ovum during transvaginal echography, the level of beta-hCG ranges from 500 to 800 mIU/ml according to the criteria of the Second International Standard, and according to the new criteria it ranges from 1,500 to 2,400 mIU/ml, which is significantly lower than the indicators which are found when compared with the results described for transabdominal echo (1,800-3,000 mIU/ml). During this period, the fertilized egg itself increases in diameter by approximately 1-2 mm daily and is determined within the thickened endometrium. To record the dynamics of the size of the fertilized egg, a repeat study can be performed after 3-5 days.

In the middle 5th week from last menstruation(3.5 weeks of gestational age), the embryo, measuring 2 to 5 mm, is located near the wall of the yolk sac, which is visualized as a round cystic structure with anechoic contents with a diameter of 2 to 4 mm. An increase in the yolk sac (over 6 mm), as well as its decrease, may indicate the death of the embryo. The embryo with the yolk sac are located eccentrically in the cavity of the fertilized egg, forming an image of the so-called “double bleb”. By the end of the first half of the embryonic period, the chorion surrounding the fertilized egg is visualized as a ring of hyperechoic tissue. At 4 weeks from the last menstruation, the size of the ovum is only 3 to 5 mm in diameter, increasing to approximately 1 cm by the 5th week of pregnancy.

In the early stages of embryo development may be difficult to visualize during transvaginal examination. Despite the fact that many of its structures have already been formed, they have not yet been identified using echography. During these times neural tube closes in the middle part, but remains open in the region of the head and caudal ends. Gill arches are formed, as well as somites, which look like rounded elevations on the surface of the embryo. Until the end of this period, 42-44 pairs of somites are formed, from which skeletal bones and muscles will subsequently develop.

The yolk sac is germinal organ, containing stock nutrients for the embryo. The yolk sac persists throughout the first trimester and resolves on its own after 12 weeks. The shape and size of the yolk sac are one of the the most important indicators the course of pregnancy in its earliest stages.

Origin

The yolk sac is formed from a special structure - the endoblastic bladder - on the 15-16th day of embryo development (or on the 29-30th day from the last menstruation). During this period, a woman may not yet be aware of her changed status, and only a delay in menstruation indicates a possible conception of a child. The yolk sac develops together with the fertilized egg and other structures of the embryo according to a program given by nature. Any deviations from the genetically programmed rhythm can lead to termination of pregnancy.

The yolk sac is a closed ring located inside the chorionic cavity. It does not function for long - only 12-14 weeks. At the beginning of the second trimester, the yolk sac begins to decrease in size. After 14 weeks, the formation disappears without a trace, having fulfilled all its functions.

Role of the yolk sac

The yolk sac is a temporary (provisional) organ, but without it the normal course of pregnancy and embryo development is impossible. In the early stages, the size of the yolk sac exceeds the size of the embryo and amniotic cavity. The yolk sac actively grows from 6 to 12 weeks of gestation, after which it gradually decreases in size and completely disappears.

On days 18-19 from conception, the yolk sac becomes the focus of hematopoiesis. In its walls, areas of erythropoiesis are formed, and the first red blood cells are formed. Subsequently, a branched network of capillaries is formed here. Primary red blood cells, leaving the yolk sac, enter the circulatory system of the embryo and are carried through the bloodstream throughout the body.

From the 28th day from the moment of conception, the yolk sac begins to produce primary germ cells of the embryo. Subsequently, the germ cells migrate from the yolk sac and reach the gonads (sex glands). 4-5 weeks of pregnancy – important stage in the development of the fetal reproductive system. Any negative impacts during this period (infections, radiation, intake medicines) can disrupt the formation of the gonads of the embryo and cause infertility.

From 2 to 6 weeks of pregnancy, the yolk sac acts as a liver for the embryo. The walls of the yolk sac synthesize important proteins and enzymes necessary for the normal development of the entire organism. In particular, AFP (alpha fetoprotein) is produced here. In the fetal circulatory system, AFP binds to PUFAs (polyunsaturated fatty acids) and transports them to all cells and tissues. AFP also suppresses the immune response to newly synthesized proteins, allowing metabolic processes to proceed at the desired rhythm.

Other functions of the yolk sac:

• regulation immune system fetus;
• synthesis of hormones;
• creating conditions for adequate metabolism;
• excretion of metabolic products.

The yolk sac performs all its functions until the main internal organs are formed in the fetal body and take over this work. After 12 weeks there is no need for a yolk sac. By the beginning of the second trimester, only a small cystic formation at the base of the umbilical cord remains from the yolk sac.

Yolk sac on ultrasound

During ultrasound examination with a transvaginal sensor, the yolk sac is detected from 6 to 12 weeks of pregnancy. Minor deviations (up to 2 weeks) in any direction are allowed. The absence of a yolk sac on ultrasound is an unfavorable sign indicating serious problems during pregnancy.

When performing an ultrasound, the doctor evaluates the location, shape and size of the yolk sac. The size of the yolk sac will depend on the gestational age.

Yolk sac norms by week:

It is important to remember: the size of the yolk sac changes rapidly in early pregnancy. Minor deviations should not frighten a pregnant woman and cannot be the basis for making serious diagnoses. If the size of the yolk sac is not normal, the doctor must carefully examine the embryo, determine the location of the fertilized egg and other parameters. If necessary, a repeat ultrasound is performed after 1-2 weeks.

Time frame for ultrasound:

• 6-7 weeks;
• 12-14 weeks.

At 6-7 weeks, the first ultrasound examination during pregnancy is performed. During the procedure, the doctor confirms the fact of pregnancy and determines its duration. The doctor indicates the location of the fertilized egg (in the uterus or outside it), assesses the condition and location of the yolk sac and chorion. The size of the fetus is determined, its correspondence to the gestational age and the size of the yolk sac. At 6 weeks, the embryo's heartbeat is also listened to and its viability is assessed.

At 12-14 weeks, the first ultrasound screening is performed. During the procedure, the doctor evaluates the condition of the embryo, chorion and yolk sac. During this period, the yolk sac reaches its maximum sizes. When performing ultrasound in more than late dates the yolk sac begins to dissolve and is not always visualized on the screen. After 14 weeks, the yolk sac is normally not detectable.

Adverse symptoms:

• absence of the yolk sac for up to 12 weeks;
• thickening of the yolk sac more than 7 mm or decrease less than 2 mm;
• change in the shape of the yolk sac.

In combination with other symptoms, these conditions may indicate a high risk of miscarriage in the first trimester. To clarify the diagnosis, additional examination using an expert-class device may be required.

Pathology of the yolk sac

When performing an ultrasound, the doctor can detect the following conditions:

The yolk sac is not visualized

Normally, the yolk sac is detected by ultrasound in the period from 6 to 12 weeks. The absence of a yolk sac is an unfavorable sign. If such an important organ is resorbed ahead of time for some reason, the embryo ceases to receive the substances necessary for its development. The synthesis of hormones and enzymes is disrupted, and the production of red blood cells stops. With premature reduction of the yolk sac (before 12 weeks), spontaneous miscarriage occurs. It is not possible to maintain pregnancy with medications.

The absence of a yolk sac on ultrasound (from 6 to 12 weeks) is considered one of the signs of a regressing pregnancy. In this case, the heartbeat of the embryo is not determined, its size does not correspond to the gestational age. Treatment is only surgical. In case of regressing pregnancy, the fertilized egg is removed and the uterine cavity is curetted.

The yolk sac is smaller than normal

Possible options:

• The yolk sac is defined as a rudimentary formation.
• The size of the yolk sac does not correspond to the duration of pregnancy (smaller than normal).

Any of these situations indicates that premature resorption of the yolk sac has begun. If at the time of reduction of the sac the internal organs of the fetus are not yet formed and are not able to fully function, the death of the embryo and spontaneous miscarriage occurs. In some cases, contractions of the uterus and miscarriage do not occur after the death of the embryo. This condition is called regressive pregnancy.

The yolk sac is larger than normal

The main reason for this symptom is incorrect determination of the gestational age. This is possible with irregular menstrual cycle(against the background of various gynecological pathologies or in nursing mothers). In this situation, the doctor should evaluate the size of the embryo and recalculate the gestational age taking into account the available data.

An important point: changes in the size, shape or density of the yolk sac are important only in combination with other ultrasound indicators. If any abnormalities are detected, the condition of the embryo (location, size, heartbeat) should be assessed. If the baby is growing and developing in accordance with the gestational age, there is no reason to worry. Changes in the yolk sac in this case are considered individual feature, which does not affect the course of the first trimester.