Oligohydramnios

Reduced amniotic fluid volume for gestational age.

Causes include:

• Placental insufficiency and FGR: fluid reduction reflects reduced fetal renal perfusion and urine output as a compensatory response to hypoxia
• Premature rupture of membranes
• Post-term pregnancy
• Fetal renal or urinary tract anomaly, particularly bilateral renal agenesis or obstructive uropathy, causing severe, early oligohydramnios
• Certain maternal medications, notably ACE inhibitors/ARBs and NSAIDs in the third trimester

Fundal height smaller than expected for gestational age; reduced fetal movements may be reported.

Severe, early-onset oligohydramnios (from significant fetal renal pathology) carries a risk of pulmonary hypoplasia given the essential role of amniotic fluid in normal lung development, and limb contractures from restricted fetal movement.

• Amniotic fluid index (AFI) below 5 cm, or single deepest vertical pocket under 2 cm, defines oligohydramnios on ultrasound
• Assessment for the underlying cause: detailed fetal anatomy and renal tract survey, umbilical artery Doppler for placental insufficiency, sterile speculum examination if membrane rupture is suspected, and maternal medication review

• Treat or address the underlying cause where identifiable: stop causative medications (ACE inhibitors/ARBs, NSAIDs), manage PPROM per that entry
• Increased fetal surveillance with serial growth scans, Doppler, and CTG, given the association with placental insufficiency and increased risk of cord compression in labor
• Amnioinfusion (instillation of fluid into the amniotic cavity, either antenatally or intrapartum via an intrauterine catheter) may be used in selected cases:
  • To reduce cord compression related fetal heart rate decelerations in labor
  • In specific research and specialist settings, to allow improved visualization for detailed anatomy scanning
• Delivery timing individualized by gestation, severity, and the underlying cause, generally earlier delivery is favored where oligohydramnios reflects significant placental insufficiency with FGR
• Severe, early, structural (renal) oligohydramnios carries a guarded prognosis and requires detailed counseling with fetal medicine and, where relevant, neonatology and pediatric urology input

Referral: fetal medicine specialist for significant or early-onset oligohydramnios, particularly where a structural fetal cause is suspected.