Polyhydramnios Secondary to Fetal Anomaly

Polyhydramnios secondary to fetal anomaly is excess amniotic fluid (amniotic fluid index 24 cm or more, or deepest vertical pocket 8 cm or more) caused by a fetal structural or genetic abnormality that reduces swallowing or raises urine or lung fluid output.

Causes include oesophageal atresia, duodenal atresia, anencephaly, neuromuscular disease, skeletal dysplasia, facial clefts and masses, diaphragmatic hernia, cardiac anomaly, hydrops, and trisomies 13, 18 and 21. It matters because the proportion with an anomaly rises with severity, and the diagnosis changes counselling, place of birth and neonatal care.

• The chance of finding an anomaly rises with the severity of polyhydramnios.
• Risk factors: abnormal anomaly scan, family history of genetic disease, maternal age above 35 years, consanguinity.
• Absent stomach bubble suggests oesophageal atresia; a double bubble suggests duodenal atresia.

Women have a uterus larger than dates, a tense abdomen, breathlessness, discomfort and leg or vulval swelling, with fetal parts hard to feel and distant heart sounds. Fetal movements may be reduced in neuromuscular disease.

• Fundal height 3 cm or more above dates.
• Rapid increase in abdominal size over 1 to 2 weeks.
• Differential diagnosis: idiopathic polyhydramnios, diabetes related polyhydramnios, multiple pregnancy, fetal anaemia, fetal infection, macrosomia, wrong dates.
• Complications: preterm labour, premature rupture of membranes, cord prolapse, malpresentation, placental abruption, postpartum haemorrhage, neonatal respiratory failure, neonatal death.

The approach is a detailed scan, with tests for diabetes, infection and chromosomal disease.

• Ultrasound: amniotic fluid index and deepest pocket; stomach size and filling over 30 to 60 minutes; bowel loops; skull, spine, face, chest, diaphragm, heart, kidneys and limbs; fetal movements and posture.
• 75 g oral glucose tolerance test: fasting 5.1 mmol/L or more, 1 hour 10.0 mmol/L or more, 2 hours 8.5 mmol/L or more.
• Maternal blood group and antibody screen, syphilis test, TORCH serology, parvovirus B19; middle cerebral artery peak systolic velocity above 1.5 multiples of the median suggests fetal anaemia.
• Amniocentesis for karyotype, microarray and infection PCR; fetal echocardiography; fetal MRI where available.

The principle is to define the anomaly, counsel the parents on outcome, relieve maternal symptoms and plan birth in a unit with neonatal surgical care.

• Supportive: refer to a fetal medicine unit; counselling on prognosis and options within the law, including care for babies with lethal anomalies; rest in a semi sitting position; teach to report contractions, leaking fluid and reduced fetal movements; birth in a facility with caesarean section and neonatal intensive care.
• Pharmacological: indomethacin 25 mg oral 6-hourly for no more than 48 hours before 32 weeks for symptomatic disease, with a repeat ultrasound of the ductus arteriosus; nifedipine 20 mg oral once then 10 to 20 mg every 6 to 8 hours for up to 48 hours for threatened preterm labour; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days if delivery is likely; magnesium sulphate 4 g IV over 20 minutes then 1 g per hour until delivery for 24 hours at most before 32 weeks; oxytocin 10 IU IM at birth then infusion of 20 IU in 1 litre sodium chloride 0.9 percent at 125 mL per hour for 4 hours; for syphilis give benzathine benzylpenicillin 2.4 million units IM once weekly for 3 weeks.
• Procedures: amnioreduction of up to 2 litres, no faster than 1 litre per 20 minutes, under ultrasound for tense symptomatic disease; induction at 37 to 38 weeks with controlled amniotomy, earlier for contractions or fetal compromise; neonatal nasogastric tube passed at birth, with surgery for oesophageal or duodenal atresia and diaphragmatic hernia.
• Special groups: lethal anomalies (anencephaly, trisomy 18, trisomy 13, severe skeletal dysplasia) need a documented plan for comfort care and a decision against operative birth for fetal reasons; women with diabetes need insulin and hourly glucose in labour.
• Red flags: contractions, rupture of membranes, breathlessness at rest, vaginal bleeding after rupture, reduced fetal movements, abdominal pain.
• Follow-up: amniotic fluid index every 1 to 2 weeks; cervical length every 2 weeks; growth scans every 4 weeks; paediatric review of the newborn with examination for the anomaly found.