Normally, the fetus in utero both urinates into and swallows the amniotic fluid. This keeps the volume of amniotic fluid constant. Polyhydramnios describes an excess of amniotic fluid in the amniotic sac. It is caused by fetal anomalies like esophageal atresia and anencephaly that result in inability to swallow the fluid, or with maternal diabetes, fetal anemia, or multiple gestations. It is diagnosed by checking the amniotic fluid, the deepest single vertical amniotic fluid pocket in each of the four uterus quadrants, via ultrasound. An index of >25 centimeters is considered polyhydramnios.