The normal umbilical cord has three vessels: two umbilical arteries and one umbilical vein. When there is a single umbilical artery, the umbilical cord has two vessels: one artery and one vein. This abnormality is present in 0.2% to 1% of pregnancies. It is more common in multiple gestations than singletons and more common in monozygotic than dizygotic twins.

Approximately 30% of fetuses with a single umbilical artery have structural anomalies; 4% have aneuploidy. Anomalies may involve any system but occur with highest frequency in the cardiovascular system, gastrointestinal tract, genitourinary tract, and central nervous system. The anomalies seen in association with a single umbilical artery can often be identified by prenatal ultrasound. Fetuses with single umbilical artery are also at increased risk for developing intrauterine growth restriction (IUGR).

Unusual variants of single umbilical artery are occasionally seen. For example, there may be two umbilical arteries in part of the umbilical cord and a single umbilical artery in the rest of the cord. In other cases, both arteries are present, but one is much smaller in diameter than the other.

The diagnosis of a single umbilical artery can be made either on a transverse view of the umbilical cord surrounded by amniotic fluid or by identifying a single umbilical artery adjacent to the bladder in the fetal pelvis with color Doppler. The transverse view of a two-vessel cord demonstrates two vessels: the larger umbilical vein and the smaller umbilical artery (Figure 22.1.1). Color Doppler will demonstrate opposite flow in the two vessels, toward the placenta in the umbilical artery and toward the fetus in the umbilical vein. A single umbilical artery can also be demonstrated with color Doppler of the fetal pelvis. The umbilical artery is seen adjacent to one side of the bladder, and no vessel is seen on the other side of the bladder (Figure 22.1.2).

Marginal and velamentous insertions of the umbilical cord into the placenta are abnormal placental cord insertions associated with increased risks to the fetus. A marginal cord insertion is one that enters the placenta within 1 cm of the placental edge. A velamentous cord insertion is present when the umbilical cord terminates at the uterine wall distant from the placenta, with the umbilical vessels traveling beneath the membranes before inserting into the placenta. Predisposing factors for these abnormal insertions include multiple gestations, low-lying placenta, placenta previa, uterine anomalies, and uterine scarring. Risks include cord rupture and thrombosis.

Marginal cord insertions are thought to develop during the course of gestation as the placenta evolves. In particular, if placental tissue on one side of the cord atrophies while placental tissue on the other side of the cord grows, the position of the cord in the placenta will be closer to one edge of the placenta than the other.

Velamentous cord insertion is thought to develop when placental tissue on one side of the cord and placental tissue beneath the cord insertion site atrophy, leaving the umbilical vessels exposed beneath the membranes as they travel to the remaining placental mass. If the vessels of a velamentous umbilical cord travel across the cervix beneath the membranes, the configuration is termed vasa previa.

Vasa previa may also result when the placenta has a succenturiate or accessory lobe. Succenturiate lobes are connected to the rest of the placenta by fetal vessels. When these connecting fetal vessels course across the cervix, a vasa previa is present.

Vasa previa carries a high risk of perinatal mortality because the fetal umbilical vessels crossing the cervix are prone to bleeding, especially if the vasa previa is not diagnosed prenatally and vaginal delivery is attempted.

Both color Doppler and grayscale ultrasound are useful for detecting and characterizing abnormal umbilical cord insertions. With a marginal cord insertion, the umbilical cord insertion into the placenta is located near the edge of the placenta (Figure 22.2.1). With velamentous cord insertion, the intra-amniotic umbilical cord is seen terminating at the uterine wall away from the placenta, and umbilical vessels travel beneath the membranes to the placenta (Figure 22.2.2).