Did you know that the bag of waters, otherwise known as the amniotic sac or the caul, is made up of two layers of membranes? Those two layers actually have layers of their own, but for the purposes of this article, we are going to tell you about the two main players in the bag: the amnion and the chorion. 

The amnion is the inner layer of the amniotic sac. In this photo, you can see it as the membrane that is visible extending from the fetal side of the placenta. Early in pregnancy, the amnion closely covers the developing embryo. At around 4 or 5 weeks, it begins to fill with amniotic fluid, which helps to expand the bag and cause it to adhere to the surface of the chorion. The volume of fluid grows until late in pregnancy, when it begins to diminish. 

The outer layer of the bag of waters is called the chorion. The chorion develops from an outer fold on the yolk sac at the very beginning of pregnancy. Early on, chorionic villi emerge from the chorion and invade the endometrium, which allows transfer of nutrients from the mother to the embryo. After about the fourth month, the chorionic villi are mostly gone, leaving the chorion smooth. In this photo, the chorion is the layer that is visible on the maternal side of the placenta.

The amnion and chorion are tightly fused to form a strong membrane that extends from the placenta to create the bag in which the fetus grows throughout pregnancy. When a mother’s “water breaks”, this is the release of amniotic fluid from a rupture in these membranes. 

When the placenta is delivered, after the birth of the infant, the bag of waters is normally fully attached to the placenta, with a small opening through which the baby passed to be born. 

Sometimes, a baby is born without the membranes rupturing at all – a baby who is born with intact waters is said to be born “in the caul”. In these cases, the amniotic sac with normally tear at some point as the baby’s body is emerging from the birth canal. During the postpartum examination of the placenta, the midwife makes certain that both the placenta and the membranes appear to be complete.