Borderline Oligohydramnios

Borderline oligohydramnios is a reduced amniotic fluid volume with an amniotic fluid index of 5.1 to 8 cm or a deepest vertical pocket of 2 to 3 cm. It may be physiological, caused by maternal dehydration, early placental insufficiency or early membrane leakage. It matters because it can progress to oligohydramnios and signal growth restriction, so follow-up scans are needed.

• Common on third trimester scans.
• Risk factors: maternal dehydration, post term pregnancy, hypertension, growth restriction, medications.
• Most women have normal outcomes.

Women usually have no symptoms. Fundal height may be slightly small, and some report reduced fetal movements or leakage of fluid.

• Normal fetal heart rate and movements in most.
• Fundal height may lag by 1 to 3 cm.
• Differential diagnosis: normal variation, dehydration, growth restriction, membrane rupture, post term placental ageing, fetal renal anomaly, ACE inhibitor or NSAID effect.
• Complications: progression to oligohydramnios, cord compression, intrapartum distress, caesarean section, stillbirth in the presence of growth restriction.

The approach is ultrasound measurement with a search for rupture, growth restriction and other causes.

• Amniotic fluid index (sum of the deepest vertical pocket in 4 quadrants) of 5.1 to 8 cm, or deepest vertical pocket of 2 to 3 cm.
• Estimated fetal weight, umbilical artery and middle cerebral artery Doppler, cerebroplacental ratio, fetal kidneys and bladder.
• Speculum examination for pooling of fluid and a test for membrane rupture (nitrazine, placental alpha microglobulin 1 or insulin-like growth factor binding protein 1 test).
• Cardiotocography; maternal blood pressure, urine protein; drug history for ACE inhibitors and NSAIDs.

The principle is hydration and repeat scanning, with delivery if fluid falls further or growth is restricted.

• Supportive: oral water 2 to 3 litres daily for 1 week; rest in the left lateral position; avoid dehydration; daily fetal movement counts; stop NSAIDs and ACE inhibitors.
• Pharmacological: sodium chloride 0.9 percent 1 litre IV over 2 hours for hydration if oral intake fails or the woman is dehydrated; labetalol 200 mg oral every 8 to 12 hours (maximum 2.4 g daily) or nifedipine modified release 30 to 60 mg oral once daily (maximum 120 mg) for hypertension; amoxicillin 500 mg oral 8-hourly for 7 days for urinary infection.
• Procedures: repeat ultrasound with amniotic fluid index and Doppler in 1 week, then weekly; induction at 39 to 40 weeks if stable, earlier for growth restriction, abnormal Doppler or reduced movements; continuous cardiotocography in labour; amnioinfusion in labour for variable decelerations.
• Red flags: reduced fetal movements, leaking fluid, falling amniotic fluid index to 5 cm or below, abnormal Doppler, abnormal cardiotocography.
• Follow-up: amniotic fluid index and Doppler weekly; cardiotocography twice weekly after 37 weeks; delivery planning at 39 weeks.