A 28-year-old primigravida with a singleton viable fetus and oligohydramnios on ultrasound—what is the most likely cause: chorioangioma, duodenal atresia, diabetes mellitus, or placental insufficiency?

Placental Insufficiency is the Most Likely Cause

In a 28-year-old primigravida with isolated oligohydramnios on ultrasound, placental insufficiency is the most likely diagnosis among the options provided.

Rationale for Placental Insufficiency (Answer D)

Placental insufficiency represents the most common pathophysiologic mechanism for oligohydramnios in singleton pregnancies without obvious fetal anomalies 1, 2. The key supporting evidence includes:

• Uteroplacental insufficiency is a well-established cause of oligohydramnios, often manifesting through fetal growth restriction, postdates pregnancy, or maternal vascular disease 3, 4
• Placental under-perfusion directly correlates with reduced amniotic fluid volume through decreased fetal renal perfusion and subsequent reduced urine production 5
• Oligohydramnios independently increases stillbirth risk 2.6-fold, reflecting the severity of placental dysfunction 6, 1, 2

Why the Other Options Are Less Likely

Chorioangioma (Option A) - Incorrect

• Chorioangiomas are rare placental tumors that typically cause polyhydramnios, not oligohydramnios 7
• When complications occur, they include polyhydramnios (most common), fetal growth restriction, and nonimmune hydrops 7
• Only 2 of 9 singleton cases with chorioangioma demonstrated oligohydramnios in one series, and these were associated with concurrent growth restriction (suggesting placental insufficiency as the mechanism) 7

Duodenal Atresia (Option B) - Incorrect

• Duodenal atresia is a gastrointestinal obstruction that classically presents with polyhydramnios, not oligohydramnios, due to impaired fetal swallowing and absorption of amniotic fluid
• While congenital anomalies can cause oligohydramnios, these are primarily genitourinary malformations (renal agenesis, severe dysplasia, or obstructive uropathy) that prevent fetal urine production 2, 3, 4

Diabetes Mellitus (Option C) - Incorrect

• Maternal diabetes is associated with polyhydramnios, not oligohydramnios 6
• Polyhydramnios occurs in diabetic pregnancies due to fetal polyuria from hyperglycemia
• While diabetes can cause placental insufficiency in advanced vascular disease, this would be the mechanism rather than diabetes itself being the direct cause

Clinical Approach to This Patient

Immediate evaluation should include:

• Detailed fetal biometry to assess for growth restriction, as oligohydramnios with IUGR indicates placental insufficiency and warrants delivery at 34-37 weeks depending on Doppler findings 1, 2
• Umbilical artery Doppler velocimetry to evaluate placental resistance, as abnormal Doppler with oligohydramnios predicts adverse outcomes and guides delivery timing 6, 1
• Comprehensive anatomic survey focusing on the genitourinary system to exclude renal anomalies as an alternative cause 3
• Maternal evaluation for hypertension, preeclampsia, or other vascular disease that could cause placental insufficiency 3

Surveillance strategy:

• Intensive antenatal testing with biophysical profile or modified BPP should begin immediately given the 2.6-fold increased stillbirth risk 6, 1
• Frequency should be twice-weekly to weekly depending on severity and associated findings 1

Critical Pitfall to Avoid

Do not assume isolated oligohydramnios is benign even without growth restriction—the independent 2.6-fold increased stillbirth risk necessitates intensive surveillance regardless of fetal size 6, 1, 2.