What is the recommended course of action for an 18-week gestation ultrasound showing a single umbilical artery (SUA)?

Management of Single Umbilical Artery at 18 Weeks Gestation

For an isolated single umbilical artery (SUA) detected at 18 weeks gestation with adequate cardiac views on the anatomy scan and negative or no prior aneuploidy screening, proceed with close monitoring during pregnancy including third-trimester growth ultrasound and weekly antenatal surveillance beginning at 36 weeks—fetal echocardiography is not routinely needed if cardiac anatomy is adequately visualized and normal. 1, 2, 3

Initial Cardiac and Anatomic Assessment

• Perform a comprehensive assessment of fetal anatomy immediately, with particular attention to cardiovascular and renal systems, as these are the most commonly affected organ systems when SUA occurs with structural abnormalities 2, 3, 4
• Cardiovascular anomalies show the strongest association with SUA, with odds ratios ranging from 5.9 to 20.3 for various cardiac defects 4, 5
• Renal anomalies demonstrate a 3-fold increased risk, occurring in approximately 16% of isolated SUA cases 4, 5

Fetal Echocardiography Decision

• If the required cardiac views are adequately visualized and normal on the anatomy scan, fetal echocardiography is NOT routinely warranted 2, 3
• This represents a critical distinction: specialist fetal echocardiography is reserved for cases where cardiac views are inadequate or abnormalities are suspected, not for all SUA cases 6
• In unselected populations with isolated SUA and no extracardiac abnormalities, the risk of major congenital heart disease is 0% (95% CI, 0-4%), making routine specialist echocardiography unnecessary 6

Genetic Testing Considerations

• No additional genetic testing is needed for isolated SUA, regardless of whether previous aneuploidy screening results were low risk or screening was declined 1, 2, 3
• Isolated SUA shows no increased risk of aneuploidy 2, 3, 4
• When SUA occurs with other structural abnormalities, aneuploidy risk ranges from 4% to 50%, but this does not apply to isolated cases 3, 4

Enhanced Surveillance Protocol

Third-Trimester Growth Monitoring

• Perform a third-trimester ultrasound examination to evaluate fetal growth, as neonates with isolated SUA have increased rates of growth restriction 1, 2, 3
• Even isolated SUA (excluding all fetuses with known anomalies) carries an adjusted odds ratio of 2.1 (95% CI 1.6-2.7) for intrauterine growth restriction 5
• Serial growth assessments should be performed at intervals of 3-4 weeks, as this provides more reliable measurements given inherent biometric error 3

Antenatal Fetal Surveillance

• Consider weekly antenatal fetal surveillance beginning at 36 weeks of gestation for fetuses with isolated SUA 2, 3
• This recommendation is based on increased risks of stillbirth (OR 4.80; 95% CI 2.67-8.62), polyhydramnios, oligohydramnios, placental abruption, cord prolapse, and perinatal mortality 2, 3, 4

If Growth Restriction Develops

• Perform serial umbilical artery Doppler assessment to evaluate for deterioration 3
• Weekly cardiotocography testing is suggested after viability if fetal growth restriction is diagnosed 3
• Delivery timing depends on Doppler findings: normal Doppler with estimated fetal weight 3rd-10th percentile warrants delivery at 38-39 weeks; decreased diastolic flow or severe growth restriction at 37 weeks; absent end-diastolic velocity at 33-34 weeks; reversed end-diastolic velocity at 30-32 weeks 3

Postnatal Considerations

• Notify the pediatric provider of the prenatal SUA findings at the time of delivery, as postnatal examination revealed structural anomalies in up to 7% of infants with prenatal diagnosis of isolated SUA 2, 3

Common Pitfalls to Avoid

• Do not order fetal echocardiography reflexively for all SUA cases—only when cardiac views are inadequate or abnormal 2, 3, 6
• Do not pursue genetic testing for isolated SUA with negative or absent prior screening, as this provides no additional benefit 1, 2, 3
• Do not provide routine antenatal care without enhanced surveillance—the nearly 5-fold increased stillbirth risk necessitates third-trimester monitoring 2, 3, 4
• Do not assume normal growth throughout pregnancy—serial assessments are essential given the 2-fold increased risk of growth restriction even in isolated cases 5