Polyhydramnios
A 36-year-old woman at 28 weeks of gestation attends for a routine scan. A single viable fetus is seen with an amniotic fluid index of 32 cm and a deepest vertical pocket of 11 cm. Fundal height measures 36 cm and she reports increasing abdominal tightness and breathlessness. Which of the following is the most likely cause of the amniotic fluid finding?
- A.Post-term pregnancy
- B.AnencephalyCorrect
- C.Posterior urethral valves
- D.Maternal indomethacin therapy
Explanation
An amniotic fluid index above 24 cm or a deepest vertical pocket above 8 cm defines polyhydramnios, and the mechanism is either excess fetal urine production or impaired fetal swallowing. Anencephaly is a neural tube defect with absence of the cranial vault and forebrain; the resulting loss of brainstem-mediated swallowing prevents clearance of amniotic fluid, so fluid accumulates. Other classic causes are oesophageal or duodenal atresia (mechanical swallowing failure), poorly controlled maternal diabetes (fetal osmotic diuresis), twin-to-twin transfusion in the recipient sac, and fetal anaemia or hydrops. In contrast, the remaining options all reduce fluid. Posterior urethral valves obstruct bladder outflow and cause oligohydramnios with a keyhole bladder and hydronephrosis. Post-term pregnancy causes declining placental function and reduced fetal urine output, so fluid falls after 41 weeks. NSAIDs such as indomethacin constrict the fetal renal afferent arteriole and reduce urine output, and they also risk premature ductal constriction. A useful rule is that oligohydramnios means low output (renal or placental) whereas polyhydramnios means too much peeing or a swallowing problem. Management of symptomatic polyhydramnios includes a detailed anomaly scan, maternal glucose testing and, if maternal distress is severe, amnioreduction.
Why each option
- A.
- Post-term pregnancy causes placental senescence with reduced fetal urine output and therefore oligohydramnios, not polyhydramnios.
- B.
- Correct. Anencephaly abolishes the fetal swallowing reflex, so amniotic fluid is not cleared and accumulates as polyhydramnios.
- C.
- Posterior urethral valves obstruct urine outflow and typically produce severe oligohydramnios with a distended bladder and hydronephrosis.
- D.
- Indomethacin reduces fetal renal blood flow and urine output and is in fact used therapeutically to treat polyhydramnios.
Reference: Williams Obstetrics, 26th ed., 2022; UpToDate 2025, Polyhydramnios: Etiology, diagnosis, and prognosis
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