The Purpose of the Yolk Sac in Pregnancy

Doctor using stethoscope on pregnant patients stomach. Credit: Caiaimage/Agnieszka Wozniak / Getty Images

Early in the first trimester of pregnancy, certain structures visible on ultrasound help provide an indication of the health of the pregnancy and developing an embryo. The first such structure is the gestational sac, which contains the baby and amniotic fluid inside the uterus and develops between three to five weeks of gestation. Soon after, in normal pregnancies, a transvaginal ultrasound performed by an experienced and trained professional will be able to visualize a yolk sac contained within the gestational sac.


Yolk Sac Features and Functions

The yolk sac nourishes the developing baby (embryo) until the circulatory system has developed. The yolk sac is visible on a transvaginal ultrasound by about five weeks of gestation, but it ultimately disappears as the baby grows.

The yolk sac is a membrane that separates from the embryo, and connected by the yolk stalk, a tube, through the umbilical opening to the embryo. Eventually, it is incorporated into the embryo's abdominal contents and replaced as a source of nutrition by the placenta.

As a pregnancy evolves, the yolk sac steadily increases between the fifth week through the end of the 10th week of gestation. After that, it gradually disappears and becomes undetectable on an ultrasound between 14 and 20 weeks of gestation. 

Yolk Sac Diameter

The yolk sac diameter (YSD) can be an important measurement in early pregnancy. In the first trimester, the absence of the yolk sac or an unusually large yolk sac may be markers for possible impending pregnancy loss.

On an ultrasound image, the yolk sac looks like a thick, circular structure within the gestational sac but separate from the amniotic membrane. It becomes visible on a transvaginal ultrasound when the mean sac diameter (MSD) reaches 8 to 10 millimeters, or on a transabdominal ultrasound when the MSD reaches 20 millimeters.


Indications of Pregnancy Health

When a yolk sac cannot be found with an ultrasound at the appropriate time of gestation, this is known as a blighted ovum, or anembryonic pregnancy, and will result in miscarriage. In many cases, this takes place before a woman has even realized she is pregnancy.

However, unlike ectopic pregnancies, in which a gestational sac is found in the uterus even though the pregnancy has implanted outside the uterus, a yolk sac found in the gestational sac confirms an intrauterine pregnancy. This rules out an ectopic pregnancy.  In addition, finding multiple yolk sacs is the earliest sign of a multiple pregnancy (twins, or greater). Generally, the number of yolk sacs will match the number of amniotic sacs in multiple pregnancies. 

However, other characteristics of the yolk sac found on ultrasound can indicate problems with the health of a pregnancy or developing embryo. For example, if the mean sac diameter (MSD) is greater than six millimeters, this raises concern about the possibility of a failed pregnancy.

When a yolk sac is found to be larger than six millimeters, generally a physician will recommend close follow-up with additional ultrasounds. In addition, when the yolk sac is smaller than expected for the number of weeks gestation, close follow-up, and additional ultrasounds may be recommended.

In addition, the shape of the yolk sac should be close to a sphere, and abnormally shaped yolk sacs raise suspicion about how the pregnancy will progress. Mainly, yolk sacs with wrinkled margins and /or indentations in the walls are classified as irregularly shaped. Some studies have found that abnormally shaped yolk sacs are linked to poorer pregnancy outcomes while others have not confirmed this. Like size, the shape of the yolk sac may prompt a physician to follow the pregnancy more closely than otherwise with additional ultrasounds. 

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