Polyhydramnios

Excess amniotic fluid volume for gestational age. Roughly 50 to 60% are idiopathic.

Identifiable causes include maternal diabetes (a leading identifiable cause, via fetal osmotic diuresis from hyperglycemia), fetal structural anomaly impairing swallowing or fluid absorption (esophageal or duodenal atresia, anencephaly, and other central nervous system or gastrointestinal anomalies), fetal anemia (from red cell alloimmunization or parvovirus B19 infection, driving high-output cardiac failure), multiple pregnancy (particularly twin-to-twin transfusion syndrome in monochorionic twins), and fetal hydrops.

Fundal height greater than expected for gestational age, maternal discomfort or breathlessness from uterine overdistension, and, clinically, increased risk of malpresentation, cord prolapse (given the increased space for the cord to slip past the presenting part with membrane rupture), preterm labor, and postpartum hemorrhage from uterine overdistension.

• Amniotic fluid index above 24 to 25 cm, or single deepest vertical pocket above 8 cm, defines polyhydramnios on ultrasound
• Screening for the underlying cause: OGTT for undiagnosed maternal diabetes, detailed fetal anatomy survey for structural anomaly, maternal red cell antibody screen and middle cerebral artery Doppler peak systolic velocity for fetal anemia, and assessment for multiple pregnancy and its specific complications

• Treat the identifiable underlying cause where present, optimize glycemic control if diabetes is diagnosed or worsened
• Mild, idiopathic polyhydramnios generally requires only increased surveillance
• Amnioreduction (therapeutic amniocentesis to drain excess fluid) is considered for severe, symptomatic polyhydramnios causing significant maternal discomfort or respiratory compromise, though fluid tends to reaccumulate and repeated procedures may be needed
• Indomethacin has historically been used to reduce fetal urine output and amniotic fluid volume in selected cases, but carries a risk of premature ductus arteriosus closure and is generally avoided beyond 32 weeks and used only under specialist guidance
• Increased vigilance for preterm labor, and, at membrane rupture (spontaneous or for induction), controlled amniotomy (slow drainage rather than rapid rupture) to reduce the risk of cord prolapse and placental abruption from sudden uterine decompression
• Active management of the third stage with vigilance for postpartum hemorrhage, given elevated risk from uterine overdistension and atony

Referral: fetal medicine specialist for moderate to severe polyhydramnios or where a structural or hematological fetal cause is identified or suspected.