A typical umbilical cord contains two arteries and one vein, but a two vessel umbilical cord has only one artery and one vein. This complication is quite rare, and it occurs slightly more often in pregnancies with multiple babies than with singletons, but it usually is harmless. In most cases, the second artery likely develops just after fertilization, just as with a regular cord, but then disappears shortly after the pregnancy begins. Although most babies with a single umbilical artery are born without complications, doctors often run extra tests throughout the pregnancy to ensure that there are no chromosomal abnormalities or birth defects.
Umbilical cord arteries are tasked with taking waste and un-oxygenated blood from the fetus back to the mother's body. Her kidneys then process and excrete the waste, keeping it away from the developing fetus. The job of the umbilical cord vein, on the other hand, is to transport oxygenated blood and nutrients from the mother to the fetus. The vein and arteries have different tasks, so normal fetal development often takes place as long as the cord contains one of each vessel. For this reason, most babies with a two vessel umbilical cord are born without complications.
Although most babies with a single umbilical artery are born without issue, many doctors prefer to run extra tests during the pregnancy, because this abnormality is often seen in stillbirths. A two vessel umbilical cord could also signal birth defects that include problems with the nervous system, urinary tract or heart. In addition, babies who have chromosomal abnormalities — in which chromosomes are missing, duplicated or in the wrong place — tend to have umbilical cord complications. Trisomy 18 is the most common chromosomal abnormality associated with a two vessel umbilical cord. The result typically is a baby with a small head, mental deficiency and clenched hands.
A two vessel umbilical cord usually is detected at a routine ultrasound during pregnancy, and after it is diagnosed, many doctors offer extra testing to the mother. The most common tests include a more detailed ultrasound, amniocentesis and echocardiography, which checks the fetal heart. In addition, many doctors recommend that the baby have an ultrasound after being born, because this can ensure that he or she does not have any birth defects that might have been missed in the womb. Some defects can be corrected before the baby is born, which is partly why the tests are run during pregnancy. In cases where defects cannot be treated, though, tests are still helpful in diagnosing problems so that the mother is aware of the issue and can prepare for it.