The amniotic sac is a tough but thin transparent pair of membranes, which hold 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. It is also called the amniotic bubble because of its resemblance to a bubble. When in the light, the amniotic sac is shiny and very smooth, but too tough to pierce through.

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, all which aid in the growth of the fetus. By the second trimester the fetus can breathe 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 healthy. Polyhydramnios is a predisposing risk factor for cord prolapse and is sometimes a side effect of a macrosomic pregnancy. Hydramnios is associated with esophageal atresia.

Preterm premature rupture of membranes (PPROM) is a condition where the amniotic sac leaks fluid before 38 weeks of gestation. This can be caused by a bacterial infection or by a defect in the structure of the amniotic sac, uterus, or cervix. In some cases, the leak can spontaneously heal, but in most cases of PPROM, labor begins within 48 hours of membrane rupture. When this occurs, it is necessary that the mother receive treatment to avoid possible infection in the newborn.

A rare and often fatal obstetric complication is an amniotic fluid embolism, which causes Disseminated intravascular coagulation.

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 blood 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.

Body fluids | Developmental biology | Embryology | Obstetrics | Pregnancy

Fruchtblase | Saco amniótico | Amnios | Amniala sako | 羊膜囊