Yolk Sac Diameter

Definition - What does Yolk Sac Diameter mean?

Yolk sac diameter is used to gauge the presence of a live pregnancy within the uterus even before the fetus can be visualized on an ultrasound. The normal yolk sac diameter is less than 6 mm. A yolk sac diameter greater than or equal to 6 mm as detected by a transvaginal ultrasound done before 12 weeks of pregnancy indicates a failed pregnancy and/or a poor pregnancy outcome.

FertilitySmarts explains Yolk Sac Diameter

The yolk sac is the first structure detected on an ultrasound within the gestational sac but outside the embryo. It serves as the key route of nutrient and gas exchange between the embryo and the mother until the development of the placental circulation. It is normally 2-5 mm in diameter. The yolk sac appears at 6 weeks of pregnancy and subsequently expands in size, attaining a maximum diameter at 10 weeks. Thereafter, it starts reducing and eventually disappears by the 12th week.

A pregnancy has a poor outcome if the yolk sac diameter is smaller or greater than the normal values for a given stage of pregnancy.

The absence of a yolk sac within a gestational sac that measures more than 20 mm as detected on an abdominal ultrasound is diagnostic of a blighted ovum. Likewise, a small yolk sac is a good predictor that the woman will likely undergo a spontaneous pregnancy loss as the fetus will not be receiving the nutrients essential for its growth.

A large yolk sac also results in nutrients being stored in the yolk sac instead of being delivered to the baby and almost always signals a poor pregnancy outcome.

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