Yolk Sac

Reviewed by Dr. Temeka Zore OB/GYN, REICheckmark
Published: July 2, 2017

What Does Yolk Sac Mean?

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.

FertilitySmarts Explains Yolk Sac

In very early pregnancy a fluid-filled pouch called a gestational sac develops within the uterus that contains 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 stalk allows blood and nutrients to circulate throughout the embryo.

What if there is no yolk sac?

Because the yolk sac plays an important role in nourishing the developing embryo, a pregnancy cannot continue to develop without one.

If a yolk sac cannot be seen on an ultrasound at 6 weeks gestation, either the pregnancy is not actually 6 weeks along or the pregnancy won't continue to develop.

To diagnosis a pregnancy loss, it is likely that an additional scan at week 7 or 8 will be done to see if the measurement of the gestation sac is 25 mm or greater and contains no embryo. This is a form of miscarriage called an anembryonic pregnancy (also called a blighted ovum), where the gestational sac forms but the embryo does not continue to grow.

What does an abnormal yolk sac mean?

There is a connection 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
  • Small yolk sac that is under 3 mm
  • Large yolk sac

What is a large yolk sac?

Large yolk sacs are linked to poor pregnancy 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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