Last revised by Jeremy Jones on 26 Sep 2021

Amnion refers to a membranous structure which covers and protects the embryo. It forms inside the chorion. The amnion usually fuses with the outer chorion by around 14 weeks of gestation.

The amnion can be visualized in most pregnancies before the 12th week of gestation and appears as a thin membrane separating the amniotic cavity, which contains the fetus, from the extra-embryonic celom and the secondary yolk sac 1.

The amniotic sac-embryo-yolk sac complex can be seen as two small blebs of almost equal size attached to the wall of the early gestational sac which is called the double bleb sign 2.

The cavity contained within the amnion is termed the amniotic cavity. Between 6.5 to 10 weeks of gestation, the length of the amniotic cavity is similar to that of the embryo 3.

"Amnion" derives from the Greek word "άμνίον", a bowl in which sacrificial blood of sheep was caught, which in turn derives from "άμνός", meaning "lamb". This is thought to be because fetal membranes were first described in sheep. The term was originally used for fetal membranes in general by Empedocles, whereas J Pollux was the first to use it to specifically refer to the amniotic membrane 5.

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