Yolk Sac

Definition - What does Yolk Sac mean?

A yolk sac is the first structure detected and contained within the gestational sac in a developing embryo. A gestational sac is a fluid-filled pouch that develops within the uterus and encloses the embryo and yolk sac. The yolk sac develops around an average of 5.5 to 6 weeks of pregnancy. The yolk sac forms outside the developing embryo and is attached to it. However, it eventually disappears by about the 12th week by being absorbed into the gut of the embryo.

The identification of a yolk sac on an ultrasound is indicative of a viable pregnancy inside the uterus (referred to as a live intrauterine pregnancy). In fact, it is the only element indicative of a live intrauterine pregnancy until an embryo can be visualized.

FertilitySmarts explains Yolk Sac

The yolk sac is connected to the midsection of the embryo’s digestive tract by a narrow tube, called the yolk stalk. After birth, the remnant of the yolk sac may turn into a small pouch attached to the small intestine, called the Meckel’s diverticulum.

The yolk sac functions to provide nourishment to the embryo during the earliest stages until the placenta takes over, which is somewhere around the 8th week of pregnancy. It also acts as a system to allow blood and nutrients to circulate throughout the embryo, called the circulatory system.

A yolk sac is only visible in a live intrauterine pregnancy, i.e. when the fetus is alive. The presence of the yolk sac and the mean diameter of the gestational sac (MSD) are assessed to determine the presence of a viable pregnancy within the uterus.

By the 7th week, if a transvaginal scan shows a mean sac diameter greater than or equal to 25 mm but no appearance of fetal pole (embryo) or yolk sac, then this indicates a failure of pregnancy, more appropriately termed as anembryonic pregnancy or blighted ovum. An anembryonic pregnancy is a pregnancy in which the gestational sac is present but it is empty and does not contain an embryo or the fetal pole measures 5mm but there is no heartbeat by the 6th week of pregnancy. This blighted ovum is not viable and aborts on its own. The woman will, however, show signs of early pregnancy due to the production of human chorionic gonadotropin (hCG) by the placenta.

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