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Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Polyhydramnios (polyhydramnion, hydramnios) is the medical condition of too much amniotic fluid in the amniotic sac. It is seen in 0.5 to 2% of pregnancies.
The opposite to Polyhydramnios is Oligohydramnios. This is a deficiency in amniotic fluid. It is very rare.
Polyhydramnios is generally due to maternal diabetes (around 20% of cases) and results from hyperglycemia of the fetus which results in fetal polyuria (fetal urine is a major source of amniotic fluid). About 20% of the cases of polyhydramnios are associated with fetal anomalies that impairs the ability of the fetus to swallow (because the fetus normally swallows amniotic fluid and absorbs it through its intestinal villi). This includes:
- gastrointestinal abnormalities such as esophageal atresia and duodenal atresia
- chromosomal abnormalities such as Down's syndrome (which is itself often associated with GI abnormalities)
- neurological abnormalities such as anencephaly, which impair the swallowing reflex
In a multiple gestation pregnancy, twin-twin transfusion syndrome is usually the cause.
Fetuses with polyhydramnios are at risk for a number of other problems including cord prolapse, placental abruption and perinatal death. At delivery the baby should be checked for congenital abnormalities.
In some cases, amnioreduction has been used in response to polyhydramnios.