Isolated Oligohydramnios

Isolated oligohydramnios is an amniotic fluid index of 5 cm or below, or a deepest vertical pocket below 2 cm, in a woman with intact membranes, a normally grown fetus, normal Doppler flows and no fetal anomaly. The cause is often not found, and it may reflect borderline placental function. It matters because it carries a modestly increased risk of intrapartum distress and stillbirth, which timed delivery lowers.

• Found at term in some otherwise normal pregnancies.
• Risk factors: post term gestation, maternal dehydration, hypertension, diabetes, smoking.
• A diagnosis of exclusion after a full work up.

Women are usually asymptomatic and normal movements are reported. Fundal height may be normal or slightly small.

• Normal fetal heart and movements.
• No leakage of fluid.
• Differential diagnosis: membrane rupture, growth restriction, renal anomaly, post term placental ageing, drug effect, twin to twin transfusion donor.
• Complications: cord compression, meconium stained liquor, intrapartum fetal distress, caesarean section, stillbirth, low Apgar score.

The approach is a full scan to exclude other causes before labelling the condition isolated.

• Ultrasound: amniotic fluid index 5 cm or below or deepest vertical pocket below 2 cm, with estimated fetal weight above the 10th centile, normal umbilical artery Doppler and cerebroplacental ratio, normal kidneys and bladder.
• Speculum examination and test for membrane rupture (nitrazine, placental alpha microglobulin 1 or insulin-like growth factor binding protein 1 test).
• Cardiotocography; maternal blood pressure, urine protein, glucose; drug history (ACE inhibitors, NSAIDs).
• Repeat scan in 1 week to confirm.

The principle is hydration, surveillance and delivery at 36 to 37 weeks and 6 days.

• Supportive: oral water 2 to 3 litres daily; left lateral rest; stop NSAIDs and ACE inhibitors; daily fetal movement counts; deliver in a facility with continuous cardiotocography and caesarean section.
• Pharmacological: sodium chloride 0.9 percent 1 litre IV over 2 hours if dehydrated; misoprostol 25 micrograms vaginal 6-hourly (maximum 4 doses) for induction in an unscarred uterus, or a Foley catheter with a 30 mL balloon if scarred; oxytocin starting at 2 mU/minute and doubling every 30 minutes to a maximum of 32 mU/minute; oxytocin 10 IU IM at birth; betamethasone 12 mg IM twice, 24 hours apart, only if delivery is needed before 35 weeks.
• Procedures: induction at 36 weeks to 37 weeks and 6 days; amnioinfusion with sodium chloride 0.9 percent 500 mL over 30 minutes then 3 mL per minute for variable decelerations in labour; continuous cardiotocography in labour; caesarean section for abnormal tracing.
• Red flags: reduced fetal movements, leakage of fluid, abnormal Doppler, abnormal cardiotocography, falling fetal growth.
• Follow-up: amniotic fluid index and Doppler twice weekly; cardiotocography twice weekly; delivery at 37 weeks.