What is the next step in management for an 18-week gestation pregnant individual with an ultrasound finding of a single umbilical artery (SUA)?

Management of Single Umbilical Artery at 18 Weeks Gestation

The correct answer is D - Close monitoring during pregnancy, which includes a comprehensive cardiac and renal assessment now, third-trimester growth ultrasound, and weekly antenatal surveillance beginning at 36 weeks. 1, 2

Immediate Assessment Required

A comprehensive assessment of cardiac anatomy should be performed immediately at the current 18-week anatomy scan. 1 The cardiovascular system is the most commonly affected when SUA occurs with structural abnormalities, with odds ratios ranging from 5.9 to 20.3 for various cardiac defects. 3 Renal anomalies show a 3-fold increased risk, with approximately 16% of isolated SUA cases having some form of renal anomaly. 3

• If the required cardiac views are adequately visualized and normal on the anatomy scan, fetal echocardiography is not routinely warranted. 1, 2
• This means option C (Fetal Echocardiography) is only necessary if the cardiac views on the anatomy scan are inadequate or abnormal. 1

Genetic Testing Not Indicated

Option B (Genetic Testing) is incorrect because isolated SUA shows no increased risk of aneuploidy. 1, 2, 3 The American College of Obstetricians and Gynecologists explicitly recommends that for fetuses with isolated SUA, no additional evaluation for aneuploidy is recommended, regardless of whether previous aneuploidy screening results were low risk or screening was declined. 2 The Society for Maternal-Fetal Medicine states that diagnostic testing for aneuploidy is not recommended solely for an isolated soft marker following negative screening. 1

• When SUA occurs with other structural abnormalities, aneuploidy risk ranges from 4% to 50%, but this does not apply to isolated SUA. 2, 3

Why Option A (Reassuring/Routine Care) is Inadequate

Option A is incorrect because isolated SUA requires enhanced surveillance beyond routine antenatal care. 1, 2 While isolated SUA has no increased aneuploidy risk, it carries significant risks for:

• Stillbirth: Nearly 5-fold increased risk (OR 4.80; 95% CI 2.67-8.62) 2, 3
• Fetal growth restriction: Increased rates requiring third-trimester monitoring 1, 2
• Other complications: Oligohydramnios, placental abruption, cord prolapse, and perinatal mortality 2

Required Surveillance Protocol

Third-Trimester Growth Assessment

• Schedule a third-trimester ultrasound examination, typically at 32-36 weeks, to evaluate fetal growth. 1, 2
• Neonates with isolated SUA have increased rates of growth restriction. 1

Weekly Antenatal Surveillance

• Begin weekly antenatal fetal surveillance at 36 0/7 weeks of gestation. 1, 2
• This is recommended due to the increased risks of stillbirth and other complications. 1

If Growth Restriction Develops

• Perform serial umbilical artery Doppler assessment to assess for deterioration. 2
• Weekly cardiotocography testing is suggested after viability if fetal growth restriction is diagnosed. 2

Postnatal Considerations

The pediatric provider should be notified of the prenatal findings at the time of delivery. 2 Postnatal examination of infants with prenatal diagnosis of isolated SUA revealed structural anomalies in up to 7% of fetuses that were not detected prenatally. 1, 2

Common Pitfalls to Avoid

• Do not dismiss SUA as benign: While isolated SUA has no increased aneuploidy risk, it requires enhanced surveillance for growth and stillbirth risk. 1, 2
• Do not routinely order fetal echocardiography: This is only needed if cardiac views on the anatomy scan are inadequate or abnormal. 1, 2
• Do not offer genetic testing: This adds unnecessary cost and anxiety without clinical benefit in isolated SUA. 1, 2