Yolk Sac

A yolk sac provides nourishment for an embryo until around 12 weeks of development when the placenta takes over. 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 is visible.

At around 5.5 weeks gestational age the yolk sac appears on transvaginal ultrasound as a round 3-5 mm structure inside a gestational sac.

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.

A large or abnormal yolk sac is associated with an increased risk of miscarriage.