Idiopathic Polyhydramnios

Idiopathic polyhydramnios is excess amniotic fluid (amniotic fluid index 24 cm or more or deepest vertical pocket 8 cm or more) with no identifiable maternal, fetal or placental cause after a full evaluation. It is the commonest category of polyhydramnios and is mostly mild. It matters because a small proportion still have an anomaly found later, and outcomes are good if a full work up is normal.

• A diagnosis of exclusion.
• Risk factors: macrosomia, family history, constitutionally large baby.
• Delivery at 39 weeks is advised.

Women have a uterus larger than dates, mild discomfort and sometimes breathlessness. Fetal parts are palpable in mild disease.

• Fundal height 2 to 6 cm above dates.
• Normal blood glucose tests.
• Differential diagnosis: diabetes related polyhydramnios, fetal swallowing disorders, fetal anaemia, infection, multiple pregnancy, chromosomal abnormality, wrong dates, macrosomia.
• Complications: preterm labour, malpresentation, cord prolapse, postpartum haemorrhage, abruption, late detection of anomalies.

The approach is a full work up that is repeated if fluid increases.

• Ultrasound: amniotic fluid index 24 cm or more or deepest vertical pocket 8 cm or more; detailed anomaly scan including stomach bubble, swallowing, kidneys, spine, skull, chest, heart and limbs; growth.
• 75 g oral glucose tolerance test (repeat at 28 to 32 weeks if first test is normal), HbA1c.
• Maternal blood group and antibody screen, syphilis, TORCH serology, parvovirus B19; middle cerebral artery peak systolic velocity.
• Amniocentesis for karyotype and microarray if fluid is moderate or severe, or anomalies are suspected.

The principle is surveillance, symptom relief and delivery at 39 weeks.

• Supportive: rest; avoid overstretching activity; counselling about preterm labour, leaking fluid and cord prolapse; delivery in a facility with caesarean section and neonatal care.
• Pharmacological: none for mild disease; indomethacin 25 mg oral 6-hourly for no more than 48 hours before 32 weeks for symptomatic moderate or severe disease, with repeat scan of the ductus arteriosus; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days if delivery is likely; oxytocin 10 IU IM at birth and infusion of 20 IU in 1 litre sodium chloride 0.9 percent at 125 mL per hour for 4 hours.
• Procedures: amnioreduction of up to 2 litres, no faster than 1 litre per 20 minutes, for symptomatic tense disease; induction at 39 weeks with controlled amniotomy; caesarean section for obstetric reasons; newborn nasogastric tube passed to exclude oesophageal atresia.
• Red flags: rapid growth in fundal height, contractions, rupture of membranes, breathlessness, reduced fetal movements.
• Follow-up: amniotic fluid index every 2 to 4 weeks; growth scans every 4 weeks; repeat glucose test at 28 to 32 weeks; newborn examination and paediatric review.