Amnion: Introduction

Amnion is the term for the innermost fetal membrane; a thin, transparent sac that holds the fetus suspended in the liquor amnii, or amniotic fluid. The amnion grows rapidly at the expense of the extraembryonic coelom, and by the end of the third month, it fuses with the chorion, forming the amniochorionic sac. It is commonly known as “the bag of waters”.