The yolk sac is a small structure attached to the embryo in early pregnancy. It provides nourishment and helps make the first blood cells before the placenta is fully formed and takes over.
The yolk sac supports the embryo during the first weeks of development. It supplies nutrients before the placenta is ready. It also helps produce the earliest blood cells and plays a part in forming the early gut. Cells that later become eggs or sperm in the baby also start near the yolk sac.
A yolk sac is usually the first structure visible inside the pregnancy sac on an early ultrasound scan, often around 5 to 6 weeks of pregnancy. Seeing it inside the womb helps confirm that the pregnancy is in the right place and is not an ectopic pregnancy. A sonographer or doctor will explain what the scan shows.
The placenta gradually takes over the job of feeding the baby during the first trimester. As this happens, the yolk sac shrinks and usually can no longer be seen on scans by the end of the first trimester. Part of it becomes incorporated into the baby's developing gut.
The yolk sac and the amniotic sac are different structures. The amniotic sac holds the fluid that surrounds and cushions the baby throughout pregnancy, while the yolk sac is a small, temporary structure used only in early development.
Sources: NHS, MedlinePlus.
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