The normal umbilical cord consists of three vessels—two arteries and one vein (Figure 109-1). Single umbilical artery (SUA) refers to the congenital absence of one of the arteries. The condition was originally described by Vesalius in 1543, Fallopio in 1561, and by Bauhin in 1621 (Persutte and Hobbins 1995). The first prenatal diagnosis of SUA was made in 1980 (Jassani et al., 1980). SUA is one of the most common malformations found in humans.

In prospective studies of liveborn infants, the incidence of SUA varied from 344 in 39,773 (0.9%) to 782 in 372,066 (0.48%) births in the United States National Collaborative Perinatal Project and a Swedish registry, respectively (Froehlich and Fujikura, 1973; Lilja, 1991). The incidence was twofold to threefold higher in a survey of spontaneous abortuses (Byrne and Blanc, 1985). In most studies, the gender distribution is equal. In the National Collaborative Perinatal Project, SUA was noted in 1.2% of white infants and 0.5% of black infants (Froehlich and Fujikura, 1973). SUA occurs three to four times more frequently among twins than among singletons (Heifetz, 1984). Other conditions associated with SUA include maternal diabetes, epilepsy, hypertension, antepartum hemorrhage, polyhydramnios, and oligohydramnios (Persutte and Hobbins, 1995). Maternal age does not affect the incidence of SUA (Prucka et al., 2004).

The normal umbilical cord contains two arteries and one vein (see Figure 109-1). SUA is easiest to demonstrate in crosssectional images (Figure 109-2) but may also be visualized longitudinally (Figure 109-3). The most reliable technique is to use color Doppler to visualize both umbilical arteries on either side of the dome of the fetal bladder. SUA can be diagnosed in the first trimester (Rembouskos et al., 2003). Fetuses with SUA have an increased diameter of the umbilical artery with no changes in diameter of the umbilical vein (Sepulveda et al., 1996b). There may, however, be a reduction in the amount of Wharton’s jelly present (Raio et al., 1999). Color flow Doppler techniques have greatly enhanced the ability to both visualize the umbilical cord (Jauniaux et al., 1989; Catanzarite et al., 1995) and measure its flow velocity waveforms (Sepulveda et al., 1996a; Ulm et al., 1997). Fetal umbilical arteries should be examined near the bifurcation of the aorta with color Doppler (see Figure 109-3). The intrafetal portion of the umbilical arteries may be easier to visualize than the free-floating cord. A SUA may have a diameter that approaches that of the umbilical vein. Diagnosis of SUA by sonography has a sensitivity of 64.9%, a specificity of 99.9%, and a positive predictive value of 64.9% (Jones et al., 1993).

Rembouskos et al. (2002) performed a prospective study in 717 consecutively examined singleton pregnancies undergoing chorionic villus sampling at 11 to 14 weeks to determine the incidence of SUA. Color flow mapping was used to visualize the umbilical arteries on either side of the bladder. The overall incidence of SUA was 5.9% (42/712), which is much higher than the liveborn incidence. In the 21 fetuses with SUA and normal chromosomes, 6 had major anomalies detected, including omphalocele, diaphragmatic hernia, megacystis, and scoliosis. In the 21 fetuses with SUA and an abnormal karyotype, 14 had trisomy 18, 5 had trisomy 21, and 2 had other abnormalities. In this study, the incidence of associated chromosome abnormalities in fetuses with SUA was much higher (50%) than in studies performed in the second and third trimester.

Nonvisualized second umbilical artery should be ruled out. False-positive diagnoses are more likely to occur before 22 weeks of gestation.

The umbilical arteries develop from the allantois, a diverticulum of the yolk sac. Between 3 and 5 weeks of gestation, a transient common umbilical artery is normally present in all embryos, replacing a plexus of arteries around the allantois. Subsequently, the common umbilical artery becomes shorter, and right and left umbilical arteries advance within the body stalk (Monie, 1970). SUA can result from one of three mechanisms: primary agenesis of one of the definitive umbilical arteries, a secondary atrophy or atresia of a previously normal umbilical artery, or persistence of the common allantoic or umbilical artery. In several prospective studies of fetuses with antenatally diagnosed SUA, the left artery was absent 69% to 73% of the time. In addition, the presence ofmultiple anomalies and/or abnormal karyotype were seen more commonly with absence of the left artery (Abuhamad et al., 1995; Geipel et al., 2000). However, other studies do not demonstrate an association between the side of the single artery and presence of malformations (Blazer et al., 1997; Budorick et al., 2001). The risk of perinatal mortality increases when SUA is diagnosed. Much of this is due to the presence of associated congenital malformations. Independent of the presence of malformations, several studies have documented an increased chance of intrauterine growth restriction (with an average birth weight of less than 2.5 kg) and preterm delivery (with an average gestation of 35.9 weeks) for infants with SUA (Heifetz, 1984; Leung and Robson, 1989; Lilja, 1991; Jones et al., 1993; Gornall et al., 2003).