About 1 percent of singleton and about 5 percent of multiple pregnancies (twins, triplets or more) have an umbilical cord that contains only two blood vessels, instead of the normal three. The cause of this abnormality, called single umbilical artery, is unknown.
The umbilical cord is the life-line that attaches the mother and developing baby. Connecting through the placenta, a normal umbilical cord is made up of three blood vessels. One large vein carries oxygen and nutrient-rich blood to the baby and the two smaller arteries carry blood and waste products back to the placenta. The umbilical cord normally grows to about two feet in length, allowing the baby enough cord to safely move around without causing damage to the cord or the placenta.
In a single umbilical artery, one artery is missing. Studies suggest that babies with single umbilical artery have an increased risk for birth defects, including heart, central nervous system and urinary-tract defects and chromosomal abnormalities. A woman whose baby is diagnosed with single umbilical artery during a routine ultrasound may be offered certain prenatal tests to diagnose or rule out birth defects. These tests may include a detailed ultrasound, amniocentesis (to check for chromosomal abnormalities) and in some cases, echocardiography (a special type of ultrasound to evaluate the fetal heart). The provider also may recommend that the baby have an ultrasound after birth.
The diagnosis of a single umbilical artery does not necessarily mean that the baby will have a birth defect. It does mean, however, that some tests are warranted and that closer attention needs to be paid for the remainder of the pregnancy.