Amniotic Fluid

The development of the amniotic sac

Inside the womb, the fetus is surrounded by a translucent, yellowish liquid called amniotic fluid. The fluid is in constant motion, swirling around the fetus and even entering its body. The fluid nourishes and protects the fetus as it develops.


The Basics

About two weeks after fertilization, the amniotic sac begins to grow and fill with fluid. Initially, the fluid is composed primarily of water. As the weeks pass, the fluid begins to fill with nutrients (such as proteins, carbohydrates and electrolytes) that aid the fetus in development. As the fetus grows, the volume of amniotic fluid increases. At about week 34 of gestation, the amniotic fluid reaches its maximum volume-about 800 mL (or about 1 quart). At full term, this number is down to about 600 mL.

The fetus "inhales" and "exhales" (swallows and excretes might be more accurate descriptions) amniotic fluid. It enters the lungs and helps them to develop. It is expelled in the form of urine.

When the Water Breaks

When a woman's "water breaks", what is actually occurring is the amniotic membrane that holds in the amniotic fluid has ruptured. Only about one in ten women actually experience a dramatic flow of amniotic fluid prior to labor. More often, the mother-to-be notices just a trickle of amniotic fluid leaking from her vagina (and it's not uncommon to notice nothing at all).

When the membrane ruptures during labor, it's called a "spontaneous rupture of membranes" (SROM). When it occurs before labor, it's called a "premature rupture of membranes" (PROM) and requires medical attention. When no spontaneous rupture occurs during labor, an "artificial rupture of membranes" (AROM), also called an amniotomy, may be performed.


Between week 15 and week 20 of pregnancy, it's common to have a doctor test the amniotic fluid. During a procedure called amniocentesis, a need is inserted into the uterus through the woman's abdomen and a small amount of amniotic fluid is withdrawn. Fetal cells are then extracted from the fluid and tested for various genetic abnormalities (such as Down syndrome and cystic fibrosis). The sample of fluid can also be used to gather information about the sex of the fetus.

Amniotic Fluid Abnormalities

Too much or too little amniotic fluid can be both an indicator of and a cause of problems with the mother and/or developing fetus. In most cases, though, a small deviation in the amount of amniotic fluid does not prevent the birth of a healthy baby.

Excess amniotic fluid is called polyhydramnios and it can occur in cases of multiple pregnancies (twins) and gestational diabetes. It can also indicate a predisposition for cord prolapse. In most cases, polyhydramnios is resolved without treatment. It is common for women who experience the problem to be monitored much more closely, however. Weekly (or even more frequent) ultrasounds are not uncommon.

Too little amniotic fluid is called oligohydramnios and it can result in deformed feet and hands. Abnormally low amniotic fluid levels can also result in a potentially fatal condition called hypoplastic lungs, in which the lungs are underdeveloped at birth. As with polyhydramnios, oligohydramnios rarely requires medical intervention.