Management of Isolated Single Umbilical Artery at 18 Weeks
For an 18-week pregnant woman with isolated single umbilical artery and no other congenital anomalies, proceed with routine antenatal care plus enhanced third-trimester surveillance—no genetic counseling, no fetal echocardiography, and absolutely no termination is indicated. 1, 2
Immediate Management: No Additional Testing Required
Genetic Counseling: NOT Indicated
- No additional evaluation for aneuploidy is recommended for isolated single umbilical artery, regardless of whether previous aneuploidy screening was low-risk or declined. 1, 2
- Isolated SUA shows no increased risk of aneuploidy when no other structural abnormalities are present 2, 3
- The Society for Maternal-Fetal Medicine explicitly states that diagnostic testing for aneuploidy is not recommended solely for an isolated soft marker following negative screening 1
Fetal Echocardiography: NOT Routinely Warranted
- If the comprehensive cardiac assessment on the anatomy scan was adequate and normal, fetal echocardiography is not routinely warranted. 2
- While cardiovascular anomalies are strongly associated with SUA (odds ratios 5.9-20.3), this applies when SUA occurs with other structural abnormalities 3
- Since no congenital anomalies were found on the anomaly scan, additional cardiac imaging beyond the standard anatomic survey is unnecessary 2
Termination: Absolutely NOT Indicated
- Termination is completely inappropriate for isolated SUA without associated anomalies 1, 2
- Research shows that 84.6% of SUA cases are isolated and have excellent outcomes, with 98.5% live birth rates 4
Required Third-Trimester Surveillance Protocol
Growth Monitoring
- Schedule a third-trimester ultrasound examination (typically 32-36 weeks) to evaluate fetal growth. 1, 2
- This is critical because neonates with isolated SUA have increased rates of growth restriction, with 9.3% weighing less than the 5th percentile 2, 4
- Even isolated SUA carries a 50% risk of growth restriction in some studies 5
Antenatal Fetal Surveillance
- Consider weekly antenatal fetal surveillance beginning at 36 0/7 weeks of gestation. 1, 2
- This recommendation is based on increased risks of stillbirth (nearly 5-fold increased risk, OR 4.80), polyhydramnios, oligohydramnios, placental abruption, and cord prolapse 2, 3
- The perinatal mortality rate is approximately 10 times greater than the general population 6
Postnatal Planning
Pediatric Notification
- Notify the pediatric team at delivery about the prenatal finding of isolated SUA. 2
- Postnatal examination reveals structural anomalies in up to 7% of fetuses with prenatal diagnosis of isolated SUA that were not detected antenatally 2
- This allows for appropriate newborn assessment and follow-up 2
Important Clinical Caveats
Why This Conservative Approach Works
The key distinction here is the word "isolated"—meaning no other structural abnormalities were detected on comprehensive anatomic survey 1, 2. When SUA occurs with other malformations, the frequency of aneuploidy ranges from 4-50%, but isolated SUA carries no increased chromosomal risk 3. The comprehensive anomaly scan already evaluated cardiac and renal anatomy (the most commonly affected systems), and these were normal 2, 3.
Common Pitfall to Avoid
Do not over-investigate isolated SUA with invasive testing or specialized imaging when the anatomy scan is normal 1, 2. The main risks with isolated SUA are growth restriction and stillbirth in the third trimester, not chromosomal abnormalities or structural defects 2, 4. Focus surveillance efforts where the actual risks lie: growth monitoring and late-pregnancy fetal well-being assessment 1, 2.