Amniotic fluid is a fluid which surrounds a developing embryo to provide nutrition, insulate the embryo from shock, and encourage healthy fetal development. Humans are often familiar with it as a symptom of labor, because the membranes which hold the fluid back usually rupture in labor, in a phenomenon which people refer to as “breaking water.” When a woman's water breaks, it means that the baby is well on the way.
12 days after conception, the amniotic sac, also known as the amnion, starts to form. This sac protects the developing fetus, and it slowly fills with amniotic fluid and swells as the fetus grows. Initially, the mother's body provides the source of liquid, but over time, much of the fluid is fetal in origin. By the end of the pregnancy, a lot of the fluid is in fact urine generated by the developing fetus.
In the early stages, the amniotic fluid is clear, and it provides critical nutrition to the fetus as it develops. Over time, it fills with shed skin cells from the fetus, along with floating stem cells. These cells can be extracted in an amniocentesis to get information about the developing fetus. By the 34th week of pregnancy, the amniotic fluid has reached its peak level, and it will start to decline in the last weeks of the pregnancy.
This liquid is similar to seawater in terms of chemical composition, and it plays a number of important roles. In the second trimester, the developing fetus drinks and breathes the amniotic fluid, with the fluid contributing to the development of healthy lungs and a digestive tract. The fluid also gives the fetus room to move around, allowing her or him to develop a healthy skeleton and strong muscles. As an interesting side note, fluids conduct sound very well, so the fluid acts like a giant amplifier, allowing the developing fetus to hear the sounds of his or her mother's body along with the outside world.
Two disorders involve amniotic fluid. In oligohydramnios, there is not enough fluid, and the fetus can develop a variety of developmental problems, including clubbed feet. In Polyhydramnios, there is too much fluid. These conditions are generally diagnosed with the use of an ultrasound, and the prognosis varies, depending on the circumstances. As a general rule, either condition merits close attention so that problems with the fetus can be identified and addressed early.