Yolk sac is a sac attached with an embryo. It can be seen in the 5th week of gestation with a size less or equal 6 mm. approximately. The sac consists of many tiny blood vessels.

Blood is transferred to the yolk sac’s wall then circulates to an embryo’s heart. So that the yolk sac provides nutrients for the embryo at early pregnancy. Once the placenta becomes fully developed, it will play a role in conveying nutrition to the embryo instead of the yolk sac later on.

From the 7th week of gestation, the embryonic development will gradually increase in terms of expanding organs while the yolk sac will remain the same size. Therefore, the yolk sac can’t provide enough food then the embryo will receive nutrients from the placenta instead.

At this stage, the placenta has already developed to serve the embryo nutrients. So that the yolk sac will finish its role and disappear.

At the end of the 12th – 14th gestational weeks, the fully developed placenta can completely replace the yolk sac in conveying food and oxygen to nourish the embryo as well as waste exchanging from the embryo’s blood to the mother.

To simply explain, yolk sac is a food source for an embryo during the early pregnancy before a placenta is completely formed then takes over its role.

MT. Jitlada Udompol
Embryologist

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Reference: Prime Fertility Center Co., Ltd.