What are the causes of oligohydramnios?

Causes of Oligohydramnios

Oligohydramnios results from disruption of normal amniotic fluid production or circulation pathways, with the most common etiologies being uteroplacental insufficiency, fetal renal abnormalities, premature rupture of membranes, and certain medications.

Fetal Renal and Urinary Tract Abnormalities

The fetal kidneys produce the majority of amniotic fluid in the second half of pregnancy through urine production, making genitourinary anomalies a primary cause of oligohydramnios 1:

• Bilateral renal agenesis (Potter syndrome) results in severe oligohydramnios and is incompatible with life due to pulmonary hypoplasia 1
• Multicystic dysplastic kidneys bilaterally prevent adequate urine production 1
• Ureteropelvic junction obstruction can reduce amniotic fluid when bilateral or severe 1
• Bladder outlet obstruction (posterior urethral valves, urethral atresia) prevents urine from entering the amniotic cavity 1
• Congenital nephrotic syndromes cause oligohydramnios through hypoproteinemia and have been associated with nonimmune hydrops fetalis 2

Uteroplacental Insufficiency

Placental dysfunction reduces fetal perfusion and subsequently decreases fetal urine output 3, 4:

• Fetal growth restriction is strongly associated with oligohydramnios, particularly when umbilical artery Doppler shows abnormal flow patterns 5
• Postterm gestation (>42 weeks) leads to placental senescence and reduced amniotic fluid production 3, 4
• Maternal hypertensive disorders compromise placental blood flow 4
• Abruptio placentae acutely reduces placental function 4

Premature Rupture of Membranes

Premature preterm rupture of membranes (PPROM) is one of the most common causes of oligohydramnios and must be clinically excluded in any case of decreased amniotic fluid 3, 1. The diagnosis is made through clinical examination showing fluid leakage, positive nitrazine or ferning tests, rather than ultrasound alone 1.

Medications

• NSAIDs after 28 weeks gestation cause oligohydramnios by reducing fetal renal function and should be avoided, particularly with administration exceeding 48 hours 5
• ACE inhibitors and angiotensin receptor blockers in the second and third trimesters impair fetal renal function 3

Twin-Specific Causes

In monochorionic diamniotic twin pregnancies, oligohydramnios in one sac requires evaluation for twin-twin transfusion syndrome 2, 5:

• Twin-twin transfusion syndrome (TTTS) is diagnosed when one twin has oligohydramnios (MVP <2 cm) and the co-twin has polyhydramnios (MVP >8 cm) 2
• The donor twin develops oligohydramnios due to hypovolemia from unbalanced placental vascular anastomoses 2
• Untreated mid-second-trimester TTTS carries >70% mortality and requires urgent referral for fetoscopic laser ablation 5

Chromosomal and Genetic Abnormalities

• Aneuploidy (particularly trisomy 18 and triploidy) can present with oligohydramnios, especially when associated with fetal anomalies 4
• Karyotype or chromosomal microarray should be performed when structural anomalies accompany oligohydramnios 5

Placental and Cord Abnormalities

Rare placental and umbilical cord lesions associated with oligohydramnios include 2:

• Umbilical cord vein thrombosis
• Umbilical vein torsion
• True knots of the umbilical cord
• Chorioangiomas (large placental tumors)

Idiopathic Oligohydramnios

Approximately 10-20% of oligohydramnios cases remain unexplained after thorough evaluation 6. These isolated cases at term still carry a 2.6-fold increased risk of stillbirth and warrant delivery consideration at 39 weeks 5.

Critical Diagnostic Approach

When oligohydramnios is identified, the evaluation must include 5, 1:

• Detailed fetal anatomic survey focusing on the genitourinary system, gastrointestinal tract, and central nervous system
• Umbilical artery Doppler velocimetry to assess for placental insufficiency
• Assessment for membrane rupture through clinical examination
• Maternal medication review for NSAIDs, ACE inhibitors, or other nephrotoxic agents
• Evaluation of fetal growth with estimated fetal weight percentiles
• Consideration of genetic testing when structural anomalies are present

The severity of oligohydramnios (defined as MVP <2 cm or AFI <5 cm) independently increases stillbirth risk with an odds ratio of 2.6, making prompt identification of the underlying cause essential for appropriate management 5, 7.