During pregnancy, the umbilical cord connects the mother and child. Blood flows through the cord from the placenta, carrying nutrients and oxygen to the fetus, and then back to the placenta with waste products from the fetus. There are typically two arteries and one vein present within the umbilical cord. In approximately 1% of pregnancies with one fetus and up to 5% of pregnancies with twins or other multiples, the vein is present but there is only a single umbilical artery.
The umbilical cord takes shape early in the pregnancy. By the fifth week, the structure is complete, and the umbilical vessels begin to function. The cord is attached to the placenta, which transfers nutrients and oxygen from the mother's blood supply to the fetal blood supply without actual mixing of the blood. The umbilical vein, the largest of the three vessels, delivers blood from the placenta to the fetus, and the arteries carry blood back to the placenta. As the arteries are smaller, the presence of a single umbilical artery (SUA) may drastically reduce the amount of waste products removed from the fetus.
A fetus with a single umbilical artery may suffer no ill effects from the condition. A two-vessel cord only indicates problems in up to 25% of diagnosed cases. When congenital problems do occur, they typically involve abnormalities at the chromosomal level or defects of the nervous system, urinary tract or heart.
Ultrasound testing during pregnancy can diagnose a single umbilical artery. If the images reveal a single umbilical vein and only one artery, the mother may be offered additional testing to determine if any congenital abnormalities may be present. These tests may include amniotic fluid sampling, known as amniocentesis, an echocardiogram of the fetal heart, or repeat or more extensive ultrasounds. Even if further testing reveals no defects, the mother may be advised to undergo periodic ultrasounds throughout the remainder of her pregnancy, to monitor blood flow through the cord and the development and health of the fetus.
As there is no known cause for the condition, there is thought to be little that can be done to prevent it. One study found a possible correlation between development of a single umbilical artery and smoking by the mother while pregnant; however, this has yet to be confirmed by additional studies. Maternal smoking is known to cause other pregnancy and birth complications, including low birth weight, problems with the placenta, and miscarriage.