A membranous sac that encloses food yolk and is attached by a tube (the yolk stalk) though the umbilical opening to the embryo’s midgut. In placental mammals (like humans) the yolk sac is nearly vestigial in later stages, as it shrinks and its job is taken over by the placenta.
The yolk sac first appears within the gestational sac during the fifth week of pregnancy and grows to be no larger than 6 mm. Yolk sacs larger than 6 mm are usually indicative of an abnormal pregnancy. Failure to identify (with transvaginal ultrasound) a yolk sac when the gestational sac has grown to 12 mm is also usually indicative of a failed pregnancy.
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