Fetal urine output in hydramnios.

Fetal urine output was evaluated in 12 pregnancies complicated by hydramnios. Six of these pregnancies were twin gestations with markedly elevated fetal urine output of the hyperperfused fetus. All six fetuses had urine output greater than the 95th percentile for their gestational age. These six patients were evaluated prior to and during the 29th week of pregnancy. Three singleton gestations had upper gastrointestinal obstruction and normal fetal urine output of 18.7, 18.0, and 12.3 mL/hour at 35, 35, and 31 weeks' gestation, respectively. Markedly elevated fetal urine output of 18 mL/hour (tenth to 95th percentile at 26 weeks, 4-7 mL/hour) was present in one fetus that was subsequently found to have diabetes insipidus. The remaining two singleton pregnancies with idiopathic hydramnios had upper limits of normal fetal urine output of 10 and 13 mL/hour at 31 and 32 weeks, respectively. It appears that hydramnios may be the result of polyuria of the hyperperfused twin in cases of twin-twin transfusion or diabetes insipidus. In upper gastrointestinal obstruction or idiopathic hydramnios, increased fetal urination does not appear to be present.