Single Umbilical Artery

Single Umbilical Artery in Short-Axis (arrow)
Color Doppler Flow Imaging of Single Umbilical Artery
Single Umbilical Artery lateral to fetal urinary bladder (arrow)


A single umbilical artery is seen in short axis adjacent to the umbilicar vein. This is also readily identified with color Doppler flow imaging of the artery in the amniotic fluid adjacent to the anterior abdominal wall and coursing lateral to the fetal urinary bladder.


The presence of two vessels within the umbilical cord (single umbilical artery) is said to be one of the most common congenital malformations. The incidence has been reported to be as low as 0.08% and as high as 1.90%. Most reviews of the subject report an incidence of fetuses with a single umbilical artery ranging from 0.2% to 1.0%. The embryology of the umbilical cord begins as early as the 3rd week with the formation of the umbilical arteries as branches of the paired dorsal aortas. The aortas eventually fuse with each other with the umbilical arteries arising from the caudal end of the aorta. The portion of the yolk sac which is not incorportated into the developing embryo will elongate into the omphalomesenteric duct, the secondary yolk sac and a diverticulum called the allantois which extends from the bladder into the body stalk. At the same time, a single artery develops around the allantois extending through the body stalk (a mesenchymal bridge connecting the embryo to the gestational sac).This artery will unite with the paired umbilical arteries at the placental insertion site. The umbilical cord thus is made up of the body stalk, the omphalomesenteric duct, the secondary yolk sac, the extra-embryonic allantois and the derived umbilical arteries and veins. During the second month of gestation the right umbilical vein regresses, resulting in two umbilical arteries and and single left umbilical vein. The paired umbilical arteries will become branches of the internal iliac arteries after growth and differentiation. They course around the fetal urinary bladder towards the umbilical cord insertion into the fetal abdomen. These vessels carry deoxygenated blood to the placenta. The persistent left umbilial vein carries oxygenated blood to the fetus, joining the left portal vein and ductus venosus eventually entering the inferior vena cava and right atrium. Umbilical cord growth continues until the end of the second trimester.

A single umbilical artery is likely due to secondary atresia or atrophy rather than primary agenesis of the artery. The left umbilical artery is absent more often than the right. One series detected an absent left umbilical artery 73% of the time vs an absent right umbilical artery 27% of the time. The side of absence can be determined by evaluating the umbilical arteries as they course around the fetal urinary bladder laterally. In fact, this is the best place to evaluate these vessels as they may fuse at the region of the umbilical cord insertion into the placenta. There are reports, however, in which two umbilical arteries were seen around the urinary bladder with only a single umbilical artery seen in the "free floating cord". In the series by Bornemeier et al this occured in 14% of their cases.

The remaining single artery is often quite large approaching the size of the umbilical vein. Sonography is often able to accurately diagnose this condition, especially with the use of color Doppler flow imaging. There are reports however, of sensitivities and positive predictive accuracies of only 65%. In addition, in one report obstetricians and pediatrician missed the diagnosis at delivery as often as 24% and 16% of cases.

2 Umbilical Arteries and the Umbilical Vein with color Doppler Flow Imaging
2 Umbilical Arteries coursing lateral to the fetal urinary bladder
A Single Umbilical Artery (arrows) which could be misinterpreted as two arteries

The first reported case of a single umbilical artery was in the early 1900' the mid 1950's it was recognized that a single umbilical artery could occur with other and often lethal malformations.30%-60% of fetuses with single umbilical arteries will have associated anomalies or chromosomal abnormalites. Virtually any organ system can be involved with an anomaly in fetuses with a single umbilical artery, however, genitourinary, cardiac and central nervous system abnormalities are the most common. Although this has not been true at all centers, a study by Abuhamad et al found complex congenital and chromosomal abnormalities exclusively when the left umbilical artery was absent.. In a study by Nyberg et al, fetuses with a known central nervous system abnormality and a single umbilical artery were found to have a significantly higher frequency of extra-CNS malformations, fetal mortality and chromosomal abnormalities than fetsuses with two umbilical arteries. In a report evaluating coexistant abnormalities with a single umbilical artery, oligohydramnios or moderate to marked polyhydramnios were univerally associated with concurrent abnormalities. As was mentioned above, chromosomal abnormalities are seen in these patients; trisomy 18 and 13 being the most common.Trisomy 21 is not commonly associated with a single umbilical artery. In addition to malformations and karyotypic abnormalities, infants with a single umbilical artery have an increased incidence of prematurity, low birth weight and intrauterine growth retardation.

There is much controversy as to what further testing should occur in patients in whom a single umbilical artery is found. It should be recognized that most studies have evaluated high risk patients (advanced maternal age, abnormal maternal serum testing, etc) and the recommendations in the literature may not apply to a low risk screened population. Most authorities agree that a targeted "survey" ultrasound should be performed, prior to decision making about amniocentesis. It is somewhat controversial as to whether a formal fetal echocardiogram should be performed in these patients. A recent study by Budorick et al found 5% of " apparently isolated single umbilical artery" fetuses had abnormal echocardiographic findings. This is much greater than the baseline risk of 0.8% to 1% in liver born infants. In their study an "apparently" isolated single umbilical artery was never associated with an abnormal karyotype. Their conclusion was that the overall increased risk of aneuploidy in the population with single umbilical arteries is attributable to the fetuses with other sonographically identified abnormalities. Their recommendation was that fetal echocardiography should be part of a workup for futher anomalies in fetuses in whom a single umbilical artery is seen.


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