Single Umbilical Artery (SUA)

A single umbilical artery (SUA) happens when the umbilical cord has only one artery instead of two. A typical umbilical cord has two arteries and one vein. Healthcare providers may call SUA a “two-vessel cord.” It occurs in 1% of all pregnancies.

There’s no treatment for SUA. And once your baby is born, the umbilical cord vessels aren’t necessary. But a single umbilical artery may affect fetal growth. It’s sometimes linked to certain congenital conditions (birth defects) and chromosomal differences.

Once your provider identifies a single umbilical artery, the most important next step is to check whether the SUA is the only finding or there are other concerns.

Types

There are two types of SUA:

• Isolated SUA: A single umbilical artery is the only difference your healthcare provider finds during an ultrasound.
• Non-isolated SUA: There are other fetal differences alongside a single umbilical artery. This may include structural differences in the fetus.

What conditions are associated with a single umbilical artery?

Fetal anomalies (birth defects) linked to a non-isolated SUA include structural differences in the:

• Urinary system (most common)
• Cardiovascular system (congenital heart disease)
• Gastrointestinal system
• Musculoskeletal system
• Central nervous system

There’s also a link between non-isolated SUA and chromosome abnormalities (aneuploidy). This happens when the number of chromosomes a cell has doesn’t equal 46. Or there are gene rearrangements, deletions or duplications. Examples include Down syndrome and trisomy 18 (Edwards syndrome). This occurs in about 10% of non-isolated SUA cases.

Isolated SUA can increase the risk of:

• Fetal growth restriction
• Preterm birth
• Lower birth weight
• Cesarean section (C-section)
• Lower Apgar score
• Needing care in the NICU

The rates of all these associated conditions and risks vary from study to study. Your healthcare provider will go over the risks with you and your testing options. They’ll give you more detailed information based on your situation. Don’t hesitate to ask questions.

How is a single umbilical artery diagnosed?

Healthcare providers can detect a single umbilical artery during a routine pregnancy ultrasound. They use color Doppler blood flow to check the blood vessels in the umbilical cord.

They can detect SUA as early as the nuchal translucency ultrasound. This exam usually happens between 11 and 13 weeks of gestation. Or your provider may find it in a later ultrasound, like the 20-week anatomy scan.

If your provider doesn’t catch the single umbilical artery during an ultrasound, they’ll see it at birth while evaluating the umbilical cord and placenta.

What tests will I need if there’s a single umbilical artery?

The first step is to determine if the SUA is an isolated finding. To do this, your provider will recommend a blood test to screen for genetic conditions and a more detailed ultrasound.

If those two tests are reassuring, it means the SUA is isolated (it’s the only finding). Your provider will likely recommend extra monitoring during your third trimester. They’ll want to check on fetal growth and well-being.

If they find abnormalities and the SUA isn’t isolated, they may recommend other tests, like amniocentesis and/or karyotype testing. These tests can definitively diagnose chromosomal or genetic conditions.

Do babies with SUA have normal outcomes?

Yes, the majority of babies with a SUA have completely normal pregnancies with additional ultrasounds. You can have a healthy pregnancy and a typical delivery and birth experience despite SUA.

In many cases, isolated SUA is simply a developmental variation (difference). Many fetuses with isolated SUA have healthy outcomes.

What causes a single umbilical artery?

Scientists don’t know exactly what causes a single umbilical artery. But they think it happens in one of the following ways during early fetal development:

• A previously normal umbilical artery closes (secondary atresia)
• A previously normal umbilical artery wastes away (atrophies)
• One of the umbilical arteries doesn’t develop at all (primary agenesis)

Research varies, but some studies show that the following factors may increase the risk of SUA:

• Having more than one fetus (multiples)
• Having diabetes before pregnancy (pregestational diabetes)
• The umbilical cord inserting into the placenta abnormally (marginal or velamentous cord insertion)