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Yolk Sac Medically Reviewed

Last Updated: June 10, 2020

Definition - What does Yolk Sac mean?

A yolk sac is a structure that provides nutritional sustenance for an embryo until around 12 weeks of development when the placenta takes over. At around 5.5 weeks gestational age it appears on ultrasound as a round 3-5 mm structure inside a gestational sac.

A yolk sac is the earliest evidence that can confirm a pregnancy is developing in the correct location inside of the uterus, usually 3-5 days before an embryo can be visible.

Evidence suggests that a large yolk sac is associated with an increased risk of miscarriage.

FertilitySmarts explains Yolk Sac

The yolk sac is the first structure identified within the gestational sac in a developing embryo. A gestational sac is a fluid-filled pouch that develops within the uterus and encloses the embryo and yolk sac.

The yolk sac is connected to the midsection of the embryo’s digestive tract by a narrow tube, called the yolk stalk.

The yolk sac provides nourishment to the embryo during the earliest stages until the placenta takes over, which is somewhere around the 12th week of pregnancy. It also acts as a system to allow blood and nutrients to circulate throughout the embryo, called the circulatory system.

Because it appears so early in pregnancy, the yolk sac is an important indicator of embryo health at the early stage. As a result, it is closely examined on ultrasounds for size and shape.

Abnormal Yolk Sac

There is a correlation between yolk sac abnormalities and early pregnancy loss. Miscarriage may occur in around 90% of cases of abnormal yolk size and around 50% of distorted yolk shapes. These abnormalities include:

  • Irregularly-shaped yolk sac that is wrinkly with indented walls
  • No yolk sac
  • Small yolk sac that is under 3 mm
  • Large yolk sac

Large Yolk Sac

Large yolk sacs are frequently related to poor outcomes. If a large yolk sac is seen on an ultrasound, follow-up imaging and close monitoring will likely be recommended.

  • Large is defined as a yolk sac diameter measuring 6 mm or greater at any gestational age.
  • A yolk sac diameter measuring over 9 mm is suggestive of a serious growth issue.
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