The yolk sac is a sac-like structure that is responsible for transporting nutrients to the embryo during pregnancy. We must not confuse it with the gestational sac. Although both are responsible for providing nutrients equally, the yolk will stop supplying its functions until the placenta has fully developed and therefore it will disappear.
The yolk sac appears within the gestational sac and it does not begin to be visualized until week 5 of gestation. Shown with a dark circular shape and can be seen through a vaginal ultrasound. It is vitally important to try to visualize its structure as the gynecologist will determine if the pregnancy is running smoothly.
What is the yolk sac?
It is a membranous annex that is attached to the embryo. It is shaped like a circular sac and dark in shape, where we can locate it within the gestational sac. It is precisely found connected to the umbilical opening of the intestine of the embryo.
This structure can be clearly seen on ultrasound and when required by a doctor. If it is visible and has a detailed shape, it will indicate that the pregnancy is proceeding normally. It will grow about 1mm per week until reaching 6 or 7mm.
What kind of role does it play?
Its function will be to provide the nutrients and oxygen necessary for the embryo to develop. But it does not only have that function, but it will produce blood cells so that the placenta is fully formed.
Much of your homework will be to eliminate the waste that is produced (including carbon dioxide), until the placenta forms and will be in charge of creating sex cells (gametes). These cells will migrate towards the primitive gonads of the embryo so that its sexual organs develop.
It is because of that it does not last throughout the pregnancy. Towards the first trimester, the life sac will disappear, since it will finish fulfilling these functions and therefore will no longer be visible by any type of ultrasound. Around week 13 is when it will disappear completely and all its functions will be replaced by the placenta.
Types of yolk sacks:
- Echogenic yolk sac: is when it can be seen correctly before an ultrasound.
- Irregular yolk sac: It is when it does not have a normal shape and indicates that the embryo is probably not developing correctly, which is why an abortion can occur.
- Calcined yolk sac: it is when there is a calcification and therefore an intrauterine death has occurred.
Depending on its form, the type of pregnancy can be diagnosed
Its shape and size by means of ultrasound they will give us the necessary indications to determine how the embryo runs its course. An abortion may occur if after the fifth week of pregnancy the sac does not reach 3mm in diameter. It can also happen if the sac is greater than 9mm, and even if it reaches 15mm it has an excessive shape and the fetus is not visualized.
Another defect is that the gestational sac together with the yolk sac are not attached to the uterus and at the top. If it is the case of being in the lower part, that is already proof that the pregnancy does not reach its end. If two yolk sacs are observed during an ultrasound, it will indicate that the pregnancy is multiple.