Physiology of amniotic fluid volume regulation.

Although there are fairly wide variations AFV normally undergoes characteristic changes across gestation in which it increases from 10-20 ml at 10 weeks gestation to average 800 ml at 24 weeks. Little change occurs from then until near term when AFV begins to decrease, and large decreases can occur in postterm pregnancies. Across gestation, 95% of AFVs are within the range of 1/2.57-2.57 times the gestational mean volume and 99% are within the range of 1/3.40-3.40 times the gestational mean. Although there are six pathways in which fluid and solutes can enter and/or leave the amniotic sac, there are only four primary pathways that contribute to AFV during late gestation. These include fetal urine and lung fluid secretion as the two primary sources of fluid, with fetal swallowing and intramembranous absorption as the two primary routes of amniotic water clearance. The intramembranous pathway also appears to be a primary source of amniotic solutes (e.g., sodium and chloride). Although fetal hypoxia has been widely believed to cause oligohydramnios, fetal hypoxic hypoxia and anemic hypoxia both appear to be associated with an increased AFV and polyhydramnios rather than oligohydramnios. It is speculated that the oligohydramnios associated with fetal hypoxia is caused by placental dysfunction in addition to the hypoxia.