Oligohydramnios

Introduction

• Description: Oligohydramnios is an abnormal reduction in the amount of amniotic fluid surrounding the fetus. At term, there should be approximately 800 mL of amniotic fluid present. Often defined as a single deepest pocket of amniotic fluid of 2 cm or less or an amniotic fluid index (sum of maximum vertical fluid pocket in each quadrant not containing umbilical cord or fetal extremities) of 5 cm or less on ultrasonography. Adequate fluid is important for fetal movement, lung development, and protection of the umbilical cord from compression.

• Prevalence: Rare in early pregnancy, common in postterm pregnancies (12%–25% at 41 weeks) and during labor after rupture of the fetal membranes.

• Predominant Age: Reproductive age.

• Genetics: No genetic pattern.

Etiology and Pathogenesis

• Causes: Generally associated with a reduction in fetal urine production (renal agenesis, urinary tract obstruction, fetal growth restriction, postterm pregnancy, and fetal death), chronic amniotic leak or preterm rupture of the membranes (35%), maternal disease (hypertension, diabetes, uteroplacental insufficiency, preeclampsia, medications).

• Risk Factors: Fetal chromosomal or congenital abnormalities (approximately 50%; see Box 243.1 ), fetal growth restriction or demise, postterm pregnancy, multiple gestation (twin–twin transfusion), maternal hypertension, diabetes, pre-eclampsia, and prostaglandin synthetase inhibitors.

  BOX 243.1

  Anomalies Associated With Oligohydramnios

  • Amniotic band syndrome

  • Cardiac anomalies: tetralogy of Fallot, septal defects

  • Central nervous system: holoprosencephaly, meningocele, encephalocele, microcephaly

  • Chromosomal: triploidy, trisomy 18, Turner syndrome

  • Cloacal dysgenesis

  • Cystic hygroma

  • Diaphragmatic hernia

  • Genitourinary tract: renal agenesis, renal dysplasia, urethral obstruction (posterior urethral valve), bladder exstrophy, Meckel-Gruber syndrome, ureteropelvic junction obstruction, prune-belly syndrome

  • Hypothyroidism

  • Multiple gestation: twin–twin transfusion syndrome, twin reverse arterial perfusion sequence (TRAP)

  • Musculoskeletal: sirenomelia, sacral agenesis, absent radius, facial clefting

  • VACTERL (vertebral, anal, cardiac, tracheoesophageal, renal, limb) association

Signs and Symptoms

• Uterine size smaller than normal for stage of pregnancy

• Reduced amniotic fluid measured by ultrasonography

Diagnostic Approach