Prevalence:

• 1 in 100 pregnancies at <24 weeks’ gestation.

Ultrasound diagnosis:

• The vertical measurement of the deepest pocket of amniotic fluid free of fetal parts is <2 cm or the amniotic fluid index (sum of vertical pockets in the four quadrants) is <5 cm.

Associated abnormalities:

• There are essentially three major causes of oligohydramnios at <24 weeks’ gestation:
  • Urinary tract abnormalities: bilateral renal agenesis, multicystic or polycystic kidneys and urethral obstruction.
  • Preterm prelabor rupture of the membranes: normal fetal growth, anatomy and fetal Doppler, with maternal history of vaginal loss of clear or blood stained amniotic fluid.
  • Uteroplacental insufficiency: Fetal growth restriction with Doppler evidence of high impedance to flow in the uterine and / or umbilical arteries and redistribution in the fetal circulation.

Investigations:

• Detailed ultrasound examination.
• In cases of unexplained oligohydramnios, amnioinfusion may be useful in allowing detailed examination of the fetus and in some cases to demonstrate that the cause was rupture of membranes.
• Invasive testing for karyotyping should be undertaken if there are relevant fetal abnormalities.

Follow up:

• Ultrasound scans every 1-3 weeks to monitor fetal condition and assess amniotic fluid volume. In cases of rupture of the membranes assessment of lung growth may be useful in predicting pulmonary hypoplasia.
• Therapeutic amnioinfusion is not useful.
• In uteroplacental insufficiency assessment of fetal growth and Doppler in the umbilical artery, ductus venosus and middle cerebral artery will help decide the best time for delivery.

Delivery:

• Fetal urinary tract abnormalities: standard obstetric care and delivery.
• Rupture of the membranes: expectant management and vaginal delivery if cephalic presentation.
• Uteroplacental insufficiency: cesarean section or vaginal delivery depending on gestational age, fetal size and degree of fetal compromise as defined by Doppler and or cardiotocography.

Prognosis:

• Depends on gestational age at diagnosis, cause and gestational age at delivery. In oligohydramnios <24 weeks’ gestation the prognosis is generally poor.
• Bilateral renal agenesis, multicystic or polycystic kidneys are lethal abnormalities, usually in the neonatal period due to pulmonary hypoplasia.
• Preterm rupture of membranes at ≤20 weeks’ gestation is associated with a poor prognosis; about 40% miscarry within 5 days of membrane rupture due to chorioamnionitis, and, in the remaining 60% of pregnancies, more than 50% of neonates die due to pulmonary hypoplasia.
• Uteroplacental insufficiency resulting in oligohydramnios at ≤24 weeks’ gestation is very severe and the most likely outcome is intrauterine death.

Recurrence:

• Renal abnormalities: agenesis or multicystic 1-3%, infantile polycystic 25%.
• Preterm rupture of membranes: 10-25%, but can be reduced by cervical cerclage and progesterone
• Uteroplacental insufficiency: 10% but can be reduced by aspirin (150 mg/day) from 12 weeks’ gestation.