Yolks do not only occur in eggs, but are also a part of the first stages of an embryo’s development. By five weeks of gestation, the yolk sac is the first part of the embryo that is visible on the ultrasound. When looking at the ultrasound during the first weeks, doctors can use the yolk sac to gauge embryonic development. The size of the yolk sac is used to determine how far along the pregnancy is. During the early pregnancy stages, a gestational sac envelops the embryo. The yolk sac is the is between the uterine wall and the embryo. Certain measurements of the yolk sac determine the progress of the pregnancy. Any irregularities in the size of the yolk sac may mean that there is a problem with probable increased risk of miscarriage or other complications.
The yolk sac provides nutrition to the embryo as it develops. It is also the first crude circulatory system for the embryo, as the placenta takes some time to fully develop and take over these functions. As the pregnancy progresses, the placenta will be the main supplier of nutrients and process the blood circulation. By the time the placenta fully takes over, the yolk sac would have depleted it’s contents.
Most of the yolk sac, however, is absorbed into the embryo as it grows. The fetus then receives nutrients from the mother. Part of the yolk sac that is not absorbed into the embryo is visible in the afterbirth, and can be shown to the mother.