Difference Between Gestational Sac and Yolk Sac (With Table)

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The gestational sac and yolk sac are two embryo-carrying sac-like structures filled with fluids that help in the maintenance of pregnancy and nourish the growing embryo. Both of them help in the growth and development of the fetus giving rise to a fully mature baby.

Gestational Sac vs. Yolk Sac

The difference between the gestational sac and yolk sac is that the gestational sac is the prior occurring sac that eventually develops to hold the yolk sac. There is a juncture in the gestational period when both the sacs exist together.

The gestational sac is the first sac-like structure that forms during the gestational period. This could be considered as the first sign of pregnancy when an ultrasound is done. This envelopes the developing embryo and contains a fluid called the amniotic fluid.

The yolk sac is found inside the gestational sac. It is the major nutrition source of the embryo before the development of the placenta. It has a yolk stalk, also called a vitelline duct that connects the yolk sac to the growing embryo, thereby aiding the functional capabilities of the yolk sac.

Comparison Table Between Gestational Sac and Yolk Sac

Parameters of ComparisonGestational SacYolk Sac  
FunctionWhen seen it is used to confirm pregnancy.Provides nutrition to the developing embryo.
Comparative SizeBiggerSmaller
Time of FormationBetween 3 and 5 weeks of gestation.Between 5.5 and 6 weeks of gestation.
AppearanceOn the ultrasound, appears as a white rim with a clear center.Appears as a dark mass with a bright rim.
PresenceCould also be present on someone who isn’t pregnant.Seen only in pregnant women.

What is Gestational Sac?

The gestational sac is one of the initial signs of pregnancy when it is visible in the ultrasound scans. It acts host to the amniotic fluid which is used in the amniocentesis process.

By this process, any kind of genetic or chromosomal abnormalities in the growing embryo could be detected before parturition, which is childbirth.

The paltry presence of the gestational sac does not for certain indicate a healthy pregnancy that will continue normally.

There are definite chances that the gestational sac might be empty, even after a few ultrasound scans.

This is a clear indication that the embryo that was supposed to develop has not even started gastrulation (early cell division). This is seen by the first trimester.

Another sign of pregnancy is high levels of HCG (human chorionic gonadotropin), which is a hormone produced by the placenta in pregnant ladies alone.

Even though the presence of HCG is evaluated to prove an existing pregnancy, sometimes the gestational sac won’t be visible in the scans. This might be the result of the following scenarios.

When ectopic pregnancies happen, gestational sac won’t be seen. Ectopic pregnancy is the one where the fertilized embryo implants in any point of the reproductive tract such as the fallopian tube other than the uterus.

These pregnancies lead to either abortion or miscarriage depending on the point of implantation.

Miscarriages are also a common situation when the gestational sac is absent. During a miscarriage, the hCG levels drop too.

The gestational sac is formed between 5 and 7 weeks after the last normal menstrual cycle.

What is Yolk Sac?

The yolk sac, also called the umbilical vesicle, is the placenta-like sac as it performs functions similar to that of the placenta.

That is, it helps in providing nutrition and also plays a role in the gas exchange of the embryo before the development of the placenta.

The yolk sac is found within the gestational sac. When the gestational sac reaches the length of 20mm approximately, the yolk sac starts its formation.

It is covered by an external layer called the extraembryonic endoderm which is in turn covered on the outer side by the extraembryonic mesoderm.

The yolk sac contributes to the formation of the umbilical cord. It is formed by the cells of the hypoblast after the embryo has been implanted.

The yolk sac disappears by the end of the first trimester and eventually becomes undetectable on an ultrasound. This is around the 14th week of gestation.

Another function performed by the yolk sac is hematopoiesis. This is the process by which the yolk sac helps produce red and white blood cells along with platelets for the fetus.

This process is continued by the yolk sac until it is taken over by the liver and bone marrow once the fetus fully develops and after parturition.

Another sac called the chorionic sac also develops from the yolk sac which plays a major role in providing cushioning and protection to the embryo against external factors.

Main Differences Between Gestational Sac and Yolk Sac

  1. While gestational sac might not necessarily be an indication of pregnancy, yolk sac, once detected in an ultrasound scan, could be considered a definite indication of pregnancy.
  2. The gestational sac houses amniotic fluid that helps in amniocentesis whereas the yolk sac has a fluid that has multiple functions, from providing nutrition to helping build the immune system of the fetus.
  3. The yolk sac is connected to the embryo with the help of the yolk stalk whereas there is no definite connecting link between the embryo and the gestational stalk.
  4. The gestational sac shows no external or internal covering or layers of protection, whereas, yolk sac has extraembryonic endoderm and mesoderm.
  5. The gestational sac can be infected due to minor ruptures on the surface in turn leading to the infection of the amniotic fluid and placenta (chorioamnionitis), while the infection of the yolk sac might make the fluid within turn brown or yellow.

Conclusion

While both the gestational and yolk sac have their importance, the yolk sac seem to have more functions while considering the well-being of the developing embryo.

Production of the yolk sac is only possible if the gestational sac matures, for the yolk sac is within the gestational sac, formed a week or so later than the gestational sac.

The absence of the gestational sac is not necessarily the absence of pregnancy, the lady might be pregnant, just an ectopic pregnancy, which in turn marks the absence of the yolk sac.

If a miscarriage happens before the formation of the yolk sac, the mother might go through excessive bleeding which eliminates the gestational sac as part of the shedding of tissues, after which she might experience normal bleeding as that during the menstrual cycle.

The yolk sac performs functions that in the long run be taken over by other organ systems of a fully matured baby after parturition.

References

  1. https://academic.oup.com/jcem/article-abstract/75/3/906/2651317
  2. https://www.nature.com/articles/nature05725

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