What is the clinical significance of a single umbilical artery (SUA) identified on obstetric ultrasound?

Clinical Significance of Single Umbilical Artery

When an isolated single umbilical artery (iSUA) is identified on prenatal ultrasound with no other structural abnormalities, no additional aneuploidy testing is needed, but third-trimester growth surveillance and weekly antenatal testing starting at 36 weeks are recommended due to increased stillbirth risk. 1

Key Distinction: Isolated vs Non-Isolated SUA

The clinical significance depends critically on whether the SUA is isolated or associated with other abnormalities:

Non-Isolated SUA (with structural abnormalities)

• Aneuploidy risk: 4-50% when other structural abnormalities are present 1
• Most commonly associated with cardiovascular and renal system abnormalities 1
• Requires comprehensive cardiac anatomy assessment (76811 ultrasound); fetal echocardiography is NOT routinely needed if cardiac views are adequately visualized and normal 1

Isolated SUA (no other abnormalities)

• No increased aneuploidy risk - additional aneuploidy evaluation is not recommended regardless of prior screening results 1
• This is the critical counseling point that distinguishes management

Perinatal Risks with Isolated SUA

Despite the absence of aneuploidy risk, isolated SUA carries significant perinatal concerns:

Stillbirth Risk

• 4.8-fold increased odds of stillbirth (OR 4.80,95% CI 2.67-8.62) 1
• This substantial mortality risk drives the surveillance recommendations

Fetal Growth Restriction (FGR)

The evidence is conflicting but concerning:

• Some studies show increased FGR risk, polyhydramnios, oligohydramnios, placental abruption, cord prolapse, and perinatal mortality 1
• Other studies found no higher-than-expected FGR incidence 1
• Recent research confirms independent association with IUGR (adjusted OR 11.3) and small-for-gestational-age (SGA) neonates 2, 3

Preterm Delivery

• 5-fold increased risk of very preterm delivery (before 34 weeks) with adjusted OR 5.0 2
• Approximately 2-fold increased risk of preterm birth overall 3

Recommended Management Algorithm

At Initial Diagnosis (Second Trimester)

1. Confirm isolation: Perform detailed anatomic survey focusing on:

   • Cardiovascular system (most common associated abnormality)
   • Renal/genitourinary system
   • Other structural abnormalities

2. Aneuploidy counseling:

   • If isolated: No additional aneuploidy testing needed 1
   • If non-isolated: Offer genetic counseling and testing given 4-50% aneuploidy risk

Third Trimester Surveillance

Perform third-trimester ultrasound to evaluate fetal growth 1

Consider weekly antenatal fetal surveillance beginning at 36 0/7 weeks 1

This recommendation is based on the conflicting FGR evidence but substantial stillbirth risk, erring on the side of caution for mortality prevention.

At Delivery

• Notify pediatric provider of prenatal findings 1
• Important because up to 7% of infants with prenatal diagnosis of isolated SUA had structural anomalies detected postnatally that were not identified prenatally 1

Additional Considerations

Incidence

• 0.25-1% of singleton pregnancies 1
• Up to 4.6% of twin gestations 1

Long-term Neurodevelopmental Outcomes

Recent evidence suggests weak associations with intellectual disability (OR 1.56) and ADHD (OR 1.22), particularly in females, though these associations are mediated less than 10% by preterm birth or SGA 4. This is reassuring regarding other neurodevelopmental disorders.

Common Pitfall

The most critical error is failing to distinguish isolated from non-isolated SUA. The presence of even one additional structural abnormality dramatically changes the aneuploidy risk from essentially zero to 4-50%, fundamentally altering management. Always perform comprehensive anatomic assessment before counseling about aneuploidy risk.