Describe polyhydramnios — its aetiology, clinical features and management.

Definition

Polyhydramnios (hydramnios) is the presence of an excessive volume of amniotic fluid (> 2000 mL). On ultrasound it is defined as an amniotic fluid index (AFI) > 25 cm or a single deepest vertical pocket > 8 cm.

Causes

• Idiopathic (the commonest, ~50%).
• Maternal — diabetes mellitus.
• Fetal — multiple pregnancy and twin-to-twin transfusion; anomalies impairing fetal swallowing or causing obstruction — anencephaly, oesophageal/duodenal atresia, neural-tube defects; and hydrops/Rh isoimmunisation.

Clinical Features

Rapid abdominal enlargement with discomfort and breathlessness; a uterus larger than dates, a tense abdomen with a fluid thrill, and difficulty in palpating fetal parts and hearing the fetal heart. Ultrasound confirms the diagnosis and screens for fetal anomalies.

Complications

Malpresentation and unstable lie, cord prolapse, preterm labour, premature rupture of membranes,

Abruptio placentae (on sudden decompression) and postpartum haemorrhage from the over-distended uterus.

Management

• Treat the cause — control maternal diabetes; investigate and manage fetal anomalies.
• Mild/moderate — observation with monitoring.
• Severe/symptomatic — amnioreduction (therapeutic amniocentesis) for relief, and indomethacin (reduces fetal urine output; avoided after 32 weeks owing to ductal constriction).
• Anticipate the above complications in labour and actively manage the third stage.

A Note on WHY Polyhydramnios Is a Clue, Not a Disease

The most useful way to think about polyhydramnios is why it should always prompt a search for a cause rather than be treated as a condition in itself. Amniotic fluid in the second half of pregnancy is maintained by a balance: the fetus produces it (as urine) and removes it (by swallowing). An excess therefore means that either the fetus is producing too much or — far more importantly — it cannot swallow. This points directly to the causes. Failure of swallowing occurs with structural obstruction of the gut (oesophageal or duodenal atresia — the classic 'double bubble') or with a neurological inability to swallow (anencephaly, other CNS anomalies, neuromuscular disorders) — so polyhydramnios is often the first hint of a serious fetal anomaly. Overproduction occurs in maternal diabetes (fetal hyperglycaemia → osmotic polyuria), in the recipient twin of twin-to-twin transfusion syndrome, and in fetal hydrops. Hence the finding demands a careful anomaly scan, a glucose tolerance test, and assessment for hydrops and congenital infection. Its own complications — maternal respiratory distress, malpresentation, cord prolapse when the membranes rupture, preterm labour, abruption on sudden decompression, and atonic PPH from the overdistended uterus — follow simply from the volume.

Look for fetal anomalies obstructing swallowing.

• Polyhydramnios = excess amniotic fluid (AFI >24 cm or deepest pool >8 cm) — always a clue to an underlying cause.
• Fetal causes (cannot swallow): oesophageal/duodenal atresia, anencephaly and other CNS/neuromuscular anomalies.
• Maternal/other causes (overproduction): diabetes, twin-to-twin transfusion (recipient), fetal hydrops, congenital infection; about a third are idiopathic.
• Investigate with a detailed anomaly scan, a glucose tolerance test and screening for hydrops/infection.
• Complications: maternal dyspnoea, malpresentation, cord prolapse, preterm labour, abruption on decompression, and atonic PPH.

A further point is that the fluid may accumulate acutely (rapidly, typically in the recipient of a twin-to-twin transfusion, causing severe maternal dyspnoea and abdominal pain and often precipitating preterm labour) or chronically (gradually, as in diabetes, where the uterus enlarges over weeks and is far better tolerated) — a distinction that determines both the urgency and the need for therapeutic amniocentesis.