Amnion: What is it? Structure, Development, Function and Amniotic Fluid

It is the membrane that covers the fetus during pregnancy.

The amnion, known as the amniotic sac , forms between the eighth and ninth day after fertilization.


The amniotic cavity is the closed sac between the embryo and the amnion, which contains the amniotic fluid.

The amniotic cavity is formed by the fusion of the parts of the amniotic fold, which first appears in the cephalic extremity and later in the caudal end and the sides of the embryo.

As the amniotic fold rises and fuses over the dorsal aspect of the embryo, the amniotic cavity forms.


At the beginning of the second week, a cavity appears within the inner cell mass and when it enlarges it becomes the amniotic cavity.

The floor of the amniotic cavity is formed by the epiblast, which migrates between the epiblastic disc and the trophoblast. In this way, epiblast cells migrate between the embryoblast and the trophoblast.

The floor is made up of the epiblast that later becomes ectoderm, while the remaining cells that are present between the embryoblasts and the trophoblast are called amnioblasts ( flattened cells ). These cells are also derived from epiblast that becomes ectoderm.

The amniotic cavity is surrounded by a membrane, called the amnion. As the implantation of the blastocyst progresses, a small space appears in the embryo, which is the primordium of the amniotic cavity.

Soon the amniotic amnioblasts (cells that form amnion) detach from the epiblast and align the amnion, which surrounds the amniotic cavity.

The epiblast forms the floor of the amniotic cavity and runs peripherally with the amnion. The hypoblast forms the roof of the exocoelomic cavity and continues with the thin exocoelomic membrane. This membrane together with the hypoblast forms the primary yolk sac.

The embryonic disc is now located between the amniotic cavity and the primary yolk sac. The cells of the yolk sac endoderm form a layer of connective tissue, the extraembryonic mesoderm, that surrounds the amnion and the yolk sac.


The amniotic sac and its filling provide a fluid that surrounds and protects the fetus. It allows the fetus to move freely within the walls of the uterus. Likewise, within the amniotic sac, is the amniotic fluid, which provides buoyancy.

Amniotic fluid

Amniotic fluid is a clear, yellow fluid found within the first 12 days after conception within the amniotic sac and surrounds the growing baby in the womb.

Amniotic fluid has many important functions and is vital for healthy fetal development.

This important liquid contains:

  • Hormones
  • Cells of the immune system.
  • Nutrients
  • Hormones
  • Your baby’s urine.

At its highest level, the amniotic fluid in your belly is about 1 quart. After 36 weeks of pregnancy, fluid levels begin to decrease as your body prepares for the delivery of your baby.