Last Updated: March 28, 2019

Definition - What does Amnion mean?

The amnion is the innermost layer of the amniotic sac, the bag of fluid that protects the embryo, and later the fetus, during pregnancy. The amnion is the location of the cells and fluid gathered during a medical procedure called an amniocentesis.

FertilitySmarts explains Amnion

The amniotic sac is composed of membranes, porous layers of cells that form a structure. There are two sections within the amniotic sac, the outer layer, called the chorion, and the inner layer called the amnion.

In the early stages of implantation, there is nothing between the amnion and the embryo. However, within the first several weeks of gestation, liquid, known as amniotic fluid, begins to develop. The amnion and amniotic sac expand as the fetus grows, providing a cushion that encases and supports the fetus.

During an amniocentesis, a physician inserts a long, thin needle through a woman's stomach and into the uterus. A sample of the liquid that surrounds the fetus, amniotic fluid, is removed for testing. The fluid is retrieved from the amnion and contains cells that provide information on the chromosomal structure and development of the fetus.

An amniocentesis is an invasive procedure. It is used only when prior testing has indicated that a genetic disorder may be present, or to monitor fetus development if pre-term labor is imminent.

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