Amniotic fluid is responsible for enlargement of the amniotic sac.
- It is a clear watery fluid that is largely derived, initially, from maternal blood plasma. Later, the fetus contributes to the volume.
- The volume increases during gestation.
- Turnover rate is about 3hrs.
- From the 5th month on the fetus swallows 400ml/day. An overabundance of amniotic fluid, polyhydramnios, may indicate maldevelopment of the fetal nervous system and the inability of the fetus to swallow.
- Fetal urine contributes to the volume. A lower than normal volume of amniotic fluid, oligohydramnios, may indicate agenesis of the fetal kidneys as one cause.
- At birth it acts as a hydrostatic wedge to help dilate the birth canal.