amniotic fluid, the fluid mixture surrounding the fetus. Its volume in a normal pregnancy increases from 250 to 800 mL between 16 and 32 weeks of gestation, remaining stable through 39 weeks and then declining to about 500 mL at 42 weeks. The origin of amniotic fluid in the first trimester is uncertain, but probably includes transudation through the chorioamnion and fetal skin before its keratinization. Its volume thereafter is maintained by a balance of resorption by fetal swallowing and flow across the membranes into the fetus and uterus and production from the fetal lungs and kidneys (fetal urine). It provides a physical barrier to trauma and a medium for active chemical exchanges, as well as facilitating fetal movement, including fetal breathing (and thereby fetal lung growth) and swallowing. The cloudy appearance of the fluid is mainly due to desquamated fetal cells and lipids. See also polyhydramnios, oligohydramnios.
