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Role of Yolk Sac in Pregnancy

Creator: Dr. P. V. Anoohya
Published: 2022-12-26T16:21:10

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The yolk sac is a tiny membrane-like structure located at the outer surface of the fetus. It consists of amniotic fluid and acts as a shield for the fetus.

Written by

Dr. P. V. Anoohya

Medically reviewed by

Dr. Richa Agarwal

December 26, 2022

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Contents

What Is the Development of the Yolk Sac?
What Is the Structure of the Yolk Sac?
What Are the Roles of the Yolk Sac in Pregnancy?
How Is Yolk Sac Useful for Pregnancy Detection?

Introduction:

The yolk sac evolves one or two weeks after the embryo is formed in the uterus. Sac is joined to the fetus with the help of the yolk stalk. The yolk stalk is connected to the midgut. Mid-gut is a derivative of the gastrointestinal system. The yolk sac substitutes the functions of the placenta before its formation in the womb. The yolk sac degenerates after three months. The yolk sac is a crucial part of the development of the fetus.

What Is the Development of the Yolk Sac?

Gastrulation is described as a developmental process in which the embryo transforms from the one-dimensional layer of epithelial cells (blastula) and rearranges into a multi-dimensional layer of cells (gastrula). Transformation of the bilaminar disc to the trilaminar disc. The end product is three primary germ layers: ectoderm, mesoderm, and endoderm. The yolk sac is endodermal in origin. Respiratory and gastrointestinal tubes originate from the endoderm. The yolk sac is inside the gestational sac, a fluid-accumulated cavity in the uterus. In turn, the yolk sac is joined on the outer side of the developing embryo. This is the exact pattern of location necessary for viable gestation. The yolk sac is not very much developed in humans compared to other mammals since it transports nutrients to the developing fetus by diffusion. Diffusion means the transport of molecules across the biological membrane without the help of any membrane proteins.

What Is the Structure of the Yolk Sac?

The yolk sac does not consist of any yolk. It passes through three stages of development:

Primary Yolk Sac:

At ten days, it forms in the blastocyst stage. Blastocyst means a rapidly dividing ball of cells. The roof is developed by the primary endoderm (hypoblast). An exocoelomic membrane creates the wall of the yolk sac.

Secondary Yolk Sac:

It develops in the chorionic vesicle stage. The roof is designed by hypoblast. The hypoblast is the endoderm of the fetus. The wall is created by an exocoelomic membrane and an inner layer of extraembryonic mesoderm.

Definitive Yolk Sac:

A portion of the yolk sac is enveloped within the fetus to form the gut. The gut is the foregut, midgut and hindgut. The bit inside the head fold is the foregut, and the portion inside the tail fold is the hindgut. The portion between the lateral folds is midgut. The leftover piece of the yolk sac that stays outside the fetus creates the definitive yolk sac. The midgut is joined to the definitive yolk sac by the yolk stalk. The yolk sac degenerates at the end of the sixth week.

What Are the Roles of the Yolk Sac in Pregnancy?

During gestation, the yolk sac is accountable for crucial biological functions.
The yolk sac contributes to the nutrition and gaseous exchange between the mother and the developing fetus.

Also, the critical organ for blood cell production.
The primary formation process of red blood cells occurs in the yolk sac before the essential organs, the liver and bone marrow, perform this function.
Stem cell immunological function is done through the yolk sac.
Macrophages are formed in the yolk sac.
Germ cell development.
Regulation of the metabolism.
Apolipoproteins, alpha-fetoprotein, and albumin proteins are developed in the yolk sac.
Umbilical cord development is through the yolk sac.
A yolk sac that is not normal is found during ultrasound scanning seven days before abortion during the first three months.
Sonography observations give the physicians a clear picture of the pregnancy and its outcomes.
The size of the yolk sac also plays a crucial role in maintaining pregnancy.
The pregnancy lasts in enlarged and prominent oval-shaped yolk sacs.
With the diminished yolk sacs, there are greater chances for abortions.
The measurement of the yolk sacs must be between three millimeters to six millimeters.
The pregnancy may lead to abortion if the yolk sac is more extensive than six millimeters.
During the scanning process, if there are numerous yolk sacs, it confirms twins or triplets.
The vascular supply of the yolk sac is called vitelline circulation.
Vitelline circulation exchange of the blood in two directions between the yolk sac and the fetus.
The yolk sac consists of a large-scale capillary network of vessels for imbibing nutrients and oxygen and supplying them to the fetus for growth.
The primary aorta carries out the function of distributing blood to the yolk sac. It is followed by capillary imbibition and drainage to vitelline veins, which carry them to the fetus.
Sometimes there is also a tumor of the yolk sac. This is a rare condition. Cancer mainly affects the cells that border the yolk sac. This tumor also occurs in children and is diagnosed in later stages of life.

How Is Yolk Sac Useful for Pregnancy Detection?

The yolk sac is the essential structure during gestation.
The visibility of the yolk sac during sonography is the indicator of pregnancy.
The gestational sac is a circular pouch around the developing fetus and the yolk sac. The gestational sac consists of fluid.
The yolk sac develops after five weeks of gestation.
If there is an absence of a yolk sac after the sixth week of scanning, it may be due to a miscalculation of dates.
In the yolk, sac calcifications complication can make it difficult to see through.

Conclusion:

Even though it is a tiny structure, the yolk sac performs critical functions and roles vital for the fetus's growth, development, and nourishment. Keeping a record of the yolk sac in the primitive stages of gestation are essential for the fetus's health. The standard size, shape, number, and texture of the yolk sac are crucial characteristics of normal pregnancy. Abnormal consistency of the yolk sac leads to early abortions. The reason is due to chromosomal abnormalities. The yolk sac cannot be assessed clinically only through scanning.

Frequently Asked Questions

1. Is the Yolk Sac Confirmatory of a Healthy Pregnancy?

The yolk sac is significant as it is the first structure visible on an ultrasound during pregnancy. Radiologists can detect the yolk sac five weeks after fertilization. The yolk sac is the essential structure during pregnancy. The visibility of the yolk sac during sonography indicates a viable (healthy) intrauterine pregnancy. The absence of a yolk sac can lead to the suspicion of a non-viable pregnancy. However, in some cases, the gestational age (the pregnancy age) is incorrect. As a result, the doctor may recommend another ultrasound in a week to look again.

2. Is Protection of the Baby a Function of the Yolk Sac?

The yolk sac carries out critical functions for fetal growth and development. It substitutes the functions of the placenta before its formation. The yolk sac performs primitive hematopoiesis (blood cell formation) and germ cell (parent cells) production during early pregnancy. Keeping a record of the yolk sac is essential for fetal health.

3. By What Process Does the Yolk Sac Provide Nutrition to the Baby?

The yolk sac fulfills many functions related to nutrition during early pregnancy. The yolk sac provides nutrition and gaseous exchange between the mother and the developing embryo. It provides nutrients through vitelline circulation (the circulation in the yolk sac). Also, the yolk sac contributes to the umbilical cord formation (it allows the transfer of oxygen and nutrients from the circulation of the mother to the fetus and removes the waste products).

4. In What Week Does the Placenta Form from the Yolk Sac?

During pregnancy, the placenta grows into an organ from a few cells. The placenta attaches to the uterine wall. It is made of both fetal and maternal tissue. By week 12, the placenta is formed and ready to nourish the fetus. However, it continues to grow throughout pregnancy and is considered mature by 34 weeks.

5. Till What Time of Pregnancy Is the Yolk Sac Visible?

The yolk sac forms during the second week of pregnancy as a round structure inside the gestational sac (a fluid-filled structure surrounding the embryo during the first few weeks of pregnancy). The yolk sac enlarges as the pregnancy progresses from week five to week 10. By week 12, it gets smaller and disappears as the embryo absorbs it. By weeks 14 to 20, it is not visible by ultrasound.

6. Till What Week of Pregnancy Does the Fetus Get Nutritional Support from the Yolk Sac?

During pregnancy, the yolk sac develops. The yolk sac changes during the first 10-12 weeks of pregnancy. It is absent during the pregnancy as it disappears after the first trimester (13 weeks). It provides nutrition and oxygen to the developing embryo for the first 12 weeks of pregnancy. The yolk sac produces the first blood cells of the embryo.

7. Does the Yolk Sac Get Nutrients from the Mother?

The human yolk sac is small and devoid of yolk. The yolk sac is essential to the embryo because of its many functions. Some evidence indicates that nutrients, such as folic acid and vitamins A, B12, and E, are concentrated in the yolk sac. Hence, the healthy development of the yolk sac requires adequate nutrition for the mother.

8. Explain the Yolk Sac and Fetal Pole.

The yolk sac is a small structure outside the embryo with various functions during pregnancy. A fetal pole is located next to the yolk sac. Both are present inside the gestational sac. The gestational sac is in the uterus in a normal pregnancy. The embryo is sometimes referred to early as the fetal pole. It usually becomes visible on an ultrasound at six weeks of pregnancy, measuring about 1-2 millimeters (mm).

9. Explain the Yolk Sac and Fetal Pole.

The yolk sac is a small structure outside the embryo with various functions during pregnancy. A fetal pole is located next to the yolk sac. Both are present inside the gestational sac. The gestational sac is in the uterus in a normal pregnancy. The embryo is sometimes referred to early as the fetal pole. It usually becomes visible on an ultrasound at six weeks of pregnancy, measuring about 1-2 millimeters (mm).

10. Does the Yolk Sac Contain the Placenta?

Humans have three fetal membranes: the yolk sac, allantois, and amnion. However, only the yolk sac and allantois form the placenta. Further, the yolk sac and the placenta provide nutrition to the fetus at different pregnancy intervals.

11. Does the Yolk Sac Develop before the Fetal Pole?

The yolk sac forms around the second week of pregnancy after implantation. Then, about six weeks after a pregnant woman's last period, a small fetal pole (one of the first stages of embryo growth) develops along the yolk sac as a thick area. However, the yolk sac reduces in size and regresses later.

12. Explain the Next Structures Formed after the Yolk Sac.

The yolk sac is a component of the gestational sac (the protective covering surrounding the embryo containing the amniotic fluid). It appears about two weeks after the embryo implantation and disappears near the end of the first trimester. It is followed by the formation of the fetal pole and the placenta at six and twelve weeks, respectively.

13. Does the Yolk Sac Burst?

After six weeks of pregnancy, an absent yolk sac during the ultrasound scan can be a sign of miscarriage. But, a follow-up scan is required to confirm no sign of fetal development. Sometimes, a gestational sac measuring 25mm and an absent yolk sac diagnose a miscarriage. However, it is unlikely that the yolk sac bursts.

14. What Does an Empty Yolk Sac Denote?

At about six weeks of pregnancy, if no yolk sac or fetal pole is visible during an ultrasound, it can be a miscarriage. Furthermore, an absent yolk sac in an eight-week pregnancy confirms the absence of an embryo. It is termed an anembryonic or empty sac pregnancy (blighted ovum). It is a miscarriage in which a medical professional advises natural termination, medical termination of pregnancy (MTP), or D&C (dilation and curettage).

15. At What hCG Level Is the Yolk Sac Visible?

Human chorionic gonadotropin (hCG) is a chemical secreted by tissue found in early embryos and the placenta later. Measuring hCG levels can help identify normal and abnormal pregnancies. The yolk sac can be first seen on the transvaginal ultrasound (TVU; done through the vagina) at hCG levels of 8000 mIU/ml (week 6 - 7). If the levels exceed 15,000 mIU/ml and the yolk sac is absent, a blighted ovum diagnosis can be given.

16. Is the Yolk Sac Implicated in Miscarriage?

Studies have shown that abnormal yolk sac characteristics are associated with miscarriage. In a normal pregnancy, one can observe the yolk sac between 5 -12 weeks or when it reaches 10 mm. Miscarriage can occur in the cases of an absent yolk sac, a yolk sac over 6 mm or under 3 mm, an irregular shape, and degenerative changes such as numerous calcifications.

17. Describe the Normal Yolk Sac Size in Pregnancy.

The yolk sac first appears at about five weeks of pregnancy and is visible on ultrasound when the mean gestational sac diameter is 5-6 mm. In a typical early pregnancy, the diameter of the yolk sac should be less than 6 mm, and the shape should be almost spherical. If a yolk sac is more than 6 mm, it may indicate a problem with the pregnancy and is associated with a poor outcome.

Sources

Dr. Richa Agarwal

Obstetrics and Gynecology

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