What causes oligohydramnios in pregnancy?

Causes of Oligohydramnios in Pregnancy

Oligohydramnios develops through multiple distinct pathophysiologic mechanisms that can be broadly categorized into fetal, maternal, placental, and iatrogenic causes, each requiring specific diagnostic evaluation and management approaches.

Fetal Causes

Genitourinary Abnormalities

• Renal agenesis (bilateral) or severe dysplasia represents the most severe fetal cause, as fetal urine production is the primary source of amniotic fluid after 16-20 weeks gestation 1, 2
• Obstructive uropathies (posterior urethral valves, ureteropelvic junction obstruction) reduce fetal urine output and amniotic fluid volume 2, 3
• A detailed anatomical survey focusing on the genitourinary tract should be performed when oligohydramnios is detected 2

Chromosomal and Genetic Abnormalities

• Chromosomal abnormalities can be associated with oligohydramnios, and karyotype should be considered in the diagnostic workup 2
• Fetal anomalies represent a significant proportion of oligohydramnios cases and require thorough ultrasound evaluation 4, 3

Growth Restriction

• Uteroplacental insufficiency leading to intrauterine growth restriction (IUGR) commonly causes oligohydramnios 2, 4, 3
• In monochorionic twins, selective IUGR (sIUGR) with oligohydramnios occurs in up to 25% of cases, with the "stuck twin" phenomenon being a pathognomonic sign when severe 5
• Oligohydramnios with IUGR warrants delivery at 34 0/7 to 37 6/7 weeks gestation depending on Doppler findings 1

Twin-Specific Causes

Twin-to-Twin Transfusion Syndrome (TTTS)

• TTTS affects 10-20% of monochorionic twins and is characterized by oligohydramnios in the donor twin (with small bladder) and polyhydramnios in the recipient twin (with large bladder) 5
• The donor twin appears as a "stuck twin" contained within collapsed membranes due to anhydramnios, which is pathognomonic for TTTS 5
• Untreated severe TTTS in mid-second trimester carries mortality exceeding 70% 5
• Severe oligohydramnios with "stuck twin" phenomenon significantly predicts mortality in growth-restricted twins with abnormal Doppler waveforms 5, 1

Maternal and Placental Causes

Maternal Medical Conditions

• Hypertension and diabetes should be thoroughly evaluated when oligohydramnios is detected, as these represent significant maternal illnesses associated with uteroplacental insufficiency 2
• Placental abruption can cause acute oligohydramnios through uteroplacental insufficiency 2, 3

Premature Rupture of Membranes (PROM)

• PROM is one of the most common causes of oligohydramnios, accounting for a significant proportion of cases 2, 4, 3
• Visualization of free amniotic bands should be attempted with ultrasound to assess for membrane rupture 2

Iatrogenic Causes

Medication-Induced

• ACE inhibitors and angiotensin receptor blockers cause fetal renal dysplasia and oligohydramnios when used in second and third trimesters, along with pulmonary hypoplasia and intrauterine growth restriction 5
• These medications should be stopped as soon as possible in the first trimester to avoid second and third trimester fetopathy 5
• NSAIDs after 28 weeks gestation can cause oligohydramnios by reducing fetal renal function and should be avoided, particularly with administration >48 hours 1

Postterm and Postdates Pregnancy

• Postterm gestation and postmaturity are associated with oligohydramnios due to declining placental function 2, 3
• Postdatism represents a form of uteroplacental insufficiency leading to decreased amniotic fluid 2

Idiopathic Oligohydramnios

• Approximately 50% of oligohydramnios cases detected on screening ultrasound are isolated with no clearly associated factors (excluding PROM, congenital anomalies, diabetes, hypertension, postdates, or IUGR) 6
• Isolated oligohydramnios at term without other complications shows no differences in Apgar scores, pH, or NICU admissions compared to normal fluid, though stillbirth risk remains 2.6-fold elevated 1, 6
• Idiopathic cases account for a significant proportion when thorough evaluation excludes identifiable causes 2

Clinical Implications

The finding of oligohydramnios independently increases stillbirth risk (odds ratio 2.6), necessitating intensive fetal surveillance regardless of etiology 1. The diagnostic workup must include assessment for hypertension, diabetes, detailed fetal anatomic survey (especially genitourinary tract), Doppler studies, and consideration of karyotype 1, 2. Maximum Vertical Pocket (MVP) measurement is preferable to Amniotic Fluid Index (AFI) as it results in fewer false-positive diagnoses and unnecessary interventions 1.