What Is a Yolk Sac in Pregnancy?

Medically Reviewed by Poonam Sachdev on October 07, 2022
4 min read

When you hear the term “yolk sac,” you might first think of a chicken egg. However, humans produce yolks as well, at least in the earliest stages of development. In fact, the yolk sac plays a vital part in ensuring a healthy and viable pregnancy.

A yolk sac is a small pouch made from thin membranes. The yolk sac is one of the first things to develop during pregnancy and one of the first things your healthcare provider will be able to see on a prenatal ultrasound. 

The yolk sac plays a very important role in early development, specifically during weeks 5 to 10, which are known as the embryonic stage. The yolk sac function includes:

  • Circulating gasses between the baby and pregnant person
  • Creating the first form of blood cells
  • Creating the cells that turn into other important structures
  • Providing the baby with nutrients
  • Providing the baby with metabolism and immune function

The yolk sac provides a baby with everything they need during the earliest stages of development. It’s also responsible for creating the cells for many of the important structures your baby will need throughout the rest of your pregnancy.

Blood cells. Blood is made up of cells, platelets, and plasma. Plasma is the liquid that transports blood cells and platelets around the body. Red blood cells carry oxygen throughout the body and return carbon dioxide to the lungs, while white blood cells are responsible for fighting off infection. Platelets cause your blood to clot.

Gastrointestinal system. The gastrointestinal system is another term for the digestive system, the system within your body that takes in food and drinks, digests them, and then excretes them. This system includes the mouth, throat, esophagus, stomach, intestines, rectum, and anus. It also includes organs and glands that produce enzymes and digestive juices that break down your food, including organs such as the gallbladder, liver, pancreas, and salivary glands.

Reproductive organs. Reproductive organs are those that contribute to reproduction. In those assigned female at birth, this includes the ovaries, fallopian tubes, uterus, cervix, and vagina. In those assigned male at birth, this includes the testes, prostate, and penis.

Umbilical cord. The umbilical cord is a tube that contains blood vessels and connects the baby to the placenta. The placenta is an organ that develops in your uterus during pregnancy and provides your baby with oxygen and nutrients. While you’re pregnant, the placenta is attached to the uterine wall, and typically, you will expel it shortly after you deliver your baby. 

The yolk sac starts to develop during the second week of pregnancy, shortly after implantation. It starts to shrink around week ten, and eventually, your baby will absorb it. Your provider will often be able to see the yolk sac via a transvaginal ultrasound by about week five. This may be before they are even able to see the fetal pole, one of the earliest forms of your baby’s embryo stage, which is usually visible via ultrasound around week six.

A typical yolk sac is round or pear-shaped. On an ultrasound, it may look like a ring with a white rim and black interior. Most yolk sacs are 3 to 5 millimeters across. Your doctor may use the size of the yolk sac to help determine how far along you are in your pregnancy. 

The yolk sac can be found within the gestational sac. The gestational sac is filled with fluid and surrounds the baby while they are in the embryo stage. The gestational sac typically implants near the top of the uterus. The uterus, sometimes referred to as the “womb,” is the organ in which the baby develops. 

Your doctor may use the appearance of the yolk sac on an ultrasound to help determine if your pregnancy is healthy and viable. 

For instance, a yolk sac that is too big (larger than six millimeters across the inside of the rim) can indicate a problem. It may be an early sign of a miscarriage, also called spontaneous abortion. A miscarriage is defined as a loss of pregnancy before the 20th week of pregnancy. Most miscarriages are outside of a pregnant person’s control. About 15% of pregnancies end in a miscarriage, and 80% of those occur in the first trimester, up to week 13 of pregnancy. 

A missing yolk sac may also indicate miscarriage, or it could indicate that you are farther along in your pregnancy than originally estimated. In this case, your provider may want to follow up with another ultrasound after a week or two. Multiple yolk sacs may indicate you are carrying more than one baby, such as twins or triplets.

If the yolk sac is oddly shaped, that may indicate a problem, but not necessarily. Your doctor will let you know if you should be concerned.

Yolk sac tumors. Yolk sac tumors, sometimes called germ cell tumors, are abnormal cells that grow on the yolk sac. After the baby is born, these cells, which often become part of the ovaries or testes, continue to grow and form a significant tumor. Most of these tumors are found in children when they are a year or two old, but some may not be discovered until later. 

Yolk sac tumors are very rare, but they may become cancerous. Tumors that appear on the reproductive organs may also cause an excess of sex hormones like testosterone or estrogen, leading to early puberty. Treatment often involves surgery, chemotherapy, or radiation depending on the location of the tumor and whether or not it has spread.