amniotic sac

The amnion is a membrane that forms a fluid-filled sac, known as the amniotic sac, which encloses an embryo in reptiles, birds, and mammals. This sac, often called the "bag of waters," develops from extra-embryonic tissue and contains fluid that cushions the embryo against mechanical injury and prevents fluid loss.1

The amniotic sac plays a crucial role in fetal development by allowing the embryo to maintain its shape and mold its body form without hindrance. It acts as a water cushion, absorbing jolts and equalizing pressure, which permits the fetus to change posture. During childbirth, the sac's fluid helps dilate the cervix.2

Abnormalities in the amount of amniotic fluid can occur. Oligohydramnios is a deficiency of amniotic fluid, which can lead to fetal distortion and developmental anomalies. Conversely, hydramnios (or polyhydramnios) is the presence of excessive amniotic fluid, often associated with fetal abnormalities and potentially causing distress to the mother.34