The fetus floats in the amniotic fluid and, in the early months of pregnancy, can move about freely. The amniotic fluid cushions the fetus against pressure from internal organs and protects it from any injury from the mother's movements.
The fluid is produced by the cells that line the amniotic sac and is constantly circulated. It is swallowed by the fetus, absorbed into its bloodstream, and then excreted by the fetal kidneys as urine. Amniotic fluid is 99 percent water. The remainder consists of dilute concentrations of the substances found in blood plasma, along with cells and lipids that have flaked off from the fetus.
Amniotic fluid appears during the first week after conception and gradually increases in volume until the tenth week when the increase becomes very rapid. By 35 weeks' gestation the volume of fluid is about one liter. Thereafter it slowly declines until it is just over half a liter at term, though there is a wide variation between individuals.
In a small number of pregnancies, excessive fluid is formed; this condition is known as polyhydramnios or hydramnios. Less frequently, insufficient amniotic fluid is formed; this condition is known as oligohydramnos.