What is the significance and recommended management of a two‑vessel (single‑artery) umbilical cord at 20 weeks gestation with normal fetal growth?

Management of Two-Vessel Cord at 20 Weeks with Normal Growth

For an isolated single umbilical artery (SUA) at 20 weeks gestation with normal fetal growth, no additional aneuploidy testing is needed regardless of prior screening status, but you should perform a third-trimester ultrasound for growth assessment and consider weekly antenatal surveillance starting at 36 weeks. 1

Immediate Assessment and Counseling

Confirm It's Truly Isolated

First, ensure this is an isolated finding by performing a comprehensive assessment of cardiac anatomy (standard 76811 ultrasound level). The most common co-occurring structural abnormalities involve the cardiovascular and renal systems 1. If the required cardiac views are adequately visualized and normal, fetal echocardiography is not routinely warranted 1.

Critical distinction: An isolated SUA carries no increased aneuploidy risk, while SUA with other structural abnormalities carries a 4-50% aneuploidy risk 1.

Aneuploidy Risk Assessment

For isolated SUA, there is no increased risk of aneuploidy 1. Therefore:

• No additional aneuploidy evaluation is recommended, even if the patient declined prior screening or had low-risk results 1
• This represents a departure from management of other soft markers
• Do not offer amniocentesis solely for this indication

Third Trimester Management

Growth Surveillance

The evidence regarding fetal growth restriction (FGR) is conflicting, but the potential for adverse outcomes warrants monitoring:

• One case-control study showed a 4.8-fold increased odds of stillbirth (OR 4.80; 95% CI 2.67-8.62) 1
• Some studies demonstrate increased risk of FGR, polyhydramnios, oligohydramnios, placental abruption, and perinatal mortality 1
• Other studies show no increased FGR incidence 1

Given this conflicting evidence but documented stillbirth risk:

• Perform a third-trimester ultrasound to evaluate fetal growth 1
• Consider weekly antenatal fetal surveillance beginning at 36 0/7 weeks gestation 1

Delivery and Postnatal Considerations

At Delivery

• Notify the pediatric provider of the prenatal SUA diagnosis 1
• Confirm the two-vessel cord at delivery

Postnatal Follow-up

Be aware that postnatal examination reveals structural anomalies in up to 7% of infants with prenatally diagnosed isolated SUA 1. This means some anomalies may not be detected on prenatal ultrasound even with careful evaluation.

Common Pitfalls to Avoid

1. False-positive diagnosis: Studies show an 8% false-positive rate for SUA diagnosis 2. Consider confirming with color flow Doppler examination of the umbilical arteries in the pelvis if there's any uncertainty.

2. Missing associated anomalies: Even when ultrasound appears normal, additional structural defects may be present that weren't visualized 3. This is why comprehensive cardiac and renal assessment is essential.

3. Over-testing for aneuploidy: Unlike other soft markers, isolated SUA does not warrant additional aneuploidy screening. Avoid unnecessary patient anxiety and testing.

4. Inadequate third-trimester surveillance: Given the documented stillbirth risk, don't skip the growth ultrasound and consideration of antenatal testing in the third trimester.

Incidence Context

SUA occurs in 0.25-1% of singleton pregnancies and up to 4.6% of twin gestations 1, making it a relatively common finding that usually has a benign course when truly isolated.