During a normal single pregnancy, the umbilical cord will contain three blood vessels: two arteries and one vein. One umbilical artery is all that is needed to handle blood flow to the placenta, so the other artery is functionally redundant.
Sometimes (between 1 and 5%) an umbilical cord may contain only two vessels, one artery and one vein. This condition is called "Single Umbilical Artery." This is the most common umbilical malformation. The cause of this abnormality is unknown. If an ultrasound examination shows that the baby appears to have no other abnormalities, the baby is likely to be born healthy.
Some studies have suggested that as many as 25% of babies with single umbilical artery may have chromosomal or other abnormalities. If your baby is diagnosed with single umbilical artery during an ultrasound, you may be offered additional testing including additional ultrasounds or even amniocentesis. Even if the baby appears to have no other abnormalities, you will probably be monitored more closely during your pregnancy. While Single Umbilical Artery may not in itself cause other abnormalities, it is often a flag that other abnormalities are present.
There is some research that indicates that single umbilical artery may contribute to poor fetal growth, low birth weight, preterm delivery and stillbirth.