The amniotic sac is a tough but thin transparent pair of membranes which holds a developing embryo (and later fetus) until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the chorion, contains the amnion and is part of the placenta.

An artificial rupture of membranes (ARM), also known as an amniotomy, may be performed by a midwife or obstetrician. This is usually performed using an amnihook and is intended to induce or accelerate labour.

Amniotic fluid

Amniotic fluid is the watery liquid surrounding and cushioning a growing fetus within the amnion. It allows the fetus to move freely without the walls of the uterus being too tight against its body. Buoyancy is also provided.

The amnion grows and begins to fill, mainly with water, around two weeks after fertilisation. After a further 10 weeks the liquid contains proteins, carbohydrates, lipids and phospholipids, urea and electrolytes. By the second trimester the fetus can breath in the water, allowing normal growth and the development of lungs and the gastrointestinal tract.

The forewaters are released when the amnion ruptures, commonly known as when a woman's "waters break" or "spontaneous rupture of membranes" (SRM). The majority of the hindwaters remain inside the womb until the baby is born.

Complications related to amniotic fluid

Too little amniotic fluid (oligohydramnios) or too much (polyhydramnios or hydramnios) can be a cause or an indicator of problems for the mother and baby. However, in both cases the majority of pregnancies proceed normally and the baby is born healthily. Polyhydramnios is a predisposing risk factor for cord prolapse.

A rare and fatal obstetric complication is an amniotic fluid embolism.

Multiple pregnancies

Twins and multiple pregnancies sometimes share the amnion and the chorion. Monoamniotic pregnancy is when each embryo or fetus from one single zygote (monozygotic, commonly known as identical twins) is located within the same amnion which is itself in one chorion (monochorionic). Diamniotic pregnancy is when there are more than one amnions inside one chorion or each having their own chorion (dichorionic). Dizygotic (fraternal, non-identical) twins each have their own amnion and chorion and may or may not share a placenta.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. Monochorionic twins, sharing one placenta, usually also share the placental bood supply. In rare cases, blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome.