Single Umbilical Artery in Multiple Gestation
The most likely cause of single umbilical artery (SUA) in this patient is her history of multiple gestation (Answer A), as SUA occurs in up to 4.6% of twin pregnancies compared to only 0.25-1% of singleton pregnancies. 1
Epidemiology and Risk Factors
The incidence of SUA is significantly elevated in multiple gestations:
- Twin pregnancies have a 4-6 fold increased risk of SUA compared to singleton pregnancies, with rates reaching up to 4.6% versus 0.25-1% in singletons 1, 2
- This elevated risk in twins is independent of maternal age, parity, or thyroid function 1
- SUA results from either primary agenesis or secondary atrophy of one umbilical artery 3
Why Multiple Gestation is the Answer
Multiple gestation itself is a direct risk factor for SUA, not merely an associated finding 1. The mechanism relates to:
- Increased placental vascular complexity in twin pregnancies 1
- Higher rates of placental vascular anastomoses and unequal placental sharing, particularly in monochorionic twins 1
- Greater likelihood of developmental vascular abnormalities when two fetuses share placental territory 1
Why Other Options Are Less Likely
Maternal age (Option B) is not established as an independent risk factor for SUA in the literature 1, 2
Parity (Option C) has no documented association with increased SUA risk 1
Hypothyroidism (Option D) is not cited as a risk factor for SUA in any of the reviewed guidelines or research 1, 2
Clinical Implications for This Patient
Given her twin history and current SUA finding:
- Perform comprehensive fetal anatomic assessment, focusing on cardiovascular and renal systems, as these are the most commonly associated anomalies 1, 2
- No additional aneuploidy testing is needed if this is an isolated SUA and prior screening was low-risk 1
- Schedule third-trimester ultrasound to evaluate fetal growth 1, 2
- Consider weekly antenatal surveillance beginning at 36 weeks gestation, as SUA carries an increased stillbirth risk (OR 4.80) 1, 2
- Monitor for growth restriction, which occurs more frequently even in isolated SUA (adjusted OR 2.1) 4