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
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Moradan S, Forouzeshfar M.
Are Abnormal Yolk Sac Characteristics Important Factors in Abortion Rates?.