A yolk sac is a structure that develops inside your uterus early in pregnancy. It provides an embryo with nutrients, circulates gasses between mother and fetus and makes cells that turn into important structures. A yolk sac’s size, appearance and location can provide important information about the health of a pregnancy.
A yolk sac is a structure that develops inside your uterus during early pregnancy. It provides an embryo with nourishment (food). It helps circulate gasses between you and the embryo. The yolk sac also produces cells that turn into important structures, such as the umbilical cord, blood cells and reproductive organs.
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The yolk sac is one of the first structures a healthcare provider can see during prenatal ultrasound. It can help confirm pregnancy, along with other factors, such as a missed menstrual cycle and levels of certain hormones in the mother’s blood.
A normal yolk sac can help confirm a healthy early pregnancy, along with other factors.
Your healthcare provider will use prenatal ultrasonography to examine the yolk sac’s:
If any of these things appear abnormal, it may mean there’s a problem with the pregnancy. Your healthcare provider may recommend another yolk sac ultrasound to recheck it in a week or two.
The yolk sac has many essential jobs in fetal development before the placenta and organs develop. The yolk sac:
The yolk sac is inside the gestational sac, a large cavity of fluid inside the uterus that surrounds an embryo. The yolk sac attaches to the outside of the developing embryo. The yolk sac must be inside the gestational sac, and the gestational sac must be inside the uterus. Otherwise, this can indicate the pregnancy has failed or will fail.
A healthy yolk sac is a round or pear-shaped pouch. It’s made of membranes (thin, skin-like material). An ultrasound may detect a fetal pole, the early form of an embryo, next to the yolk sac.
The yolk sac begins to develop during the second week of gestation (pregnancy). A healthcare provider can see the yolk sac using transvaginal ultrasound starting at about week five. The yolk sac grows as pregnancy progresses from week five to week 10.
After week 10, the yolk sac gradually gets smaller and disappears. The embryo absorbs it. By weeks 14 to 20, it may not be visible by ultrasound at all.
If your healthcare provider can’t see the yolk sac, it might indicate the pregnancy is not viable. That means the pregnancy won’t result in a birth. But it also can mean that the estimated gestational age is wrong.
Your healthcare provider may recommend another ultrasound in a week or two to look again.
In a typical early pregnancy, a yolk sac’s diameter is usually 3 to 5 millimeters from one inside rim to the other.
If a yolk sac is larger than 6 millimeters, it might indicate a problem with the pregnancy. It might be an early sign of miscarriage.
If a yolk sac is smaller than a healthcare provider expects, it might mean the gestational age was miscalculated. In other words, you may not be as far along as you thought. Your healthcare provider may recommend a follow-up ultrasound. They will recheck the yolk sac size and look for other signs that the pregnancy is healthy or not.
If an ultrasound detects more than one yolk sac, you may have multiple embryos, such as twins or triplets.
A yolk sac also can be irregular. For example, it may be shaped abnormally, or it may be hardened on areas along the outside or all over the sac.
An abnormal yolk sac might indicate a problem with the pregnancy, but it might not. Your healthcare provider will talk to you about any abnormal findings, what they mean for you and what other tests you might need.
A yolk sac tumor is a rare type of cancer, sometimes called a germ cell tumor. The cancer starts in the cells that line the yolk sac and forms a tumor sometime after birth.
This type of tumor often occurs in the ovaries or testes, but it can appear elsewhere in your body. The condition usually is diagnosed in children but can be diagnosed later in life. Germ cell tumors can produce substances (hormones) that can lead to certain changes in our bodies as we grow.
A note from Cleveland Clinic
A yolk sac is a structure that develops early in pregnancy. It nourishes the embryo and helps the embryo develop. The yolk sac is one of the earliest structures a healthcare provider can see with prenatal ultrasound. Your provider will help you understand what the yolk sac’s size, shape and appearance mean for your pregnancy.
Last reviewed by a Cleveland Clinic medical professional on 02/03/2022.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy