Single umbilical artery

Single umbilical artery


The umbilical cord forms between days 13 and 38 after conception. The umbilical arteries originate from the left and right common iliac arteries. When secondary atrophy or atresia of an umbilical artery occurs, a single umbilical artery (SUA) remains. Support for the theory of umbilical artery atrophy is derived from embryo studies that reveal only a 0.1% incidence of single umbilical arteries in embryos, in contrast to a much larger prevalence later in gestation. Secondary atrophy of an umbilical artery has also been found in several cases of less than 20 weeks’ gestation. The method by which the umbilical cord is examined affects the reported incidence of a two vessel umbilical cord. Fujikura reported a 0.54% incidence of SUA by gross examination and a 0.86% incidence when the umbilical cord was examined microscopically. While the overall incidence of a SUA is approximately 1%, the incidence of a SUA varies in different sub-populations. For example, the incidence of a SUA in fetuses after a spontaneous miscarriage, fetuses with a significant malformation, and fetuses with a karyotypic abnormality is 1.5%, 7%, and 8.5, respectively. A single umbilical artery occurs 3 to 4 times more frequently in twin pregnancies. There is 9.3% incidence of velamentous cord insertion with a SUA. The number of vessels within the umbilical cord is most easily assessed on a transverse image of the umbilical cord. Several loops of umbilical cord should be evaluated before determining the number of vessels within the umbilical cord. A convergence of the two umbilical arteries within the umbilical cord may lead to individual transverse images of either a two vessel or three vessel umbilical cord. The umbilical arteries may also converge either within 10 to 15cm of its placental end or intra-abdominally. Fusion of the umbilical arteries does not have the same prognostic significance as a complete two vessel umbilical cord. In some cases it is difficult to obtain an image that accurately identifies all 3 vessels. In a study of 17,777 consecutive singleton births, Jones et al reported a 65% sensitivity and positive predictive value for the sonographic detection of a single umbilical artery. The 3.5-fold increase in the prematurity rate with a single umbilical artery is due to the associated malformations. Intrauterine growth restriction occurs twice as frequently among fetuses with a SUA when compared to fetuses with two umbilical arteries. It has been hypothesized that a failure of the remaining umbilical artery to dilate may result in intrauterine growth restriction. The placentas of SUA infants weigh significantly less than control placentas (p < 0.05). The perinatal mortality for growth restricted fetuses with a SUA is significantly higher than for growth restricted fetuses with a three vessel umbilical cord. The rate of stillbirth is increased, not only in the anomalous fetuses with a two vessel umbilical cord, but also in those fetuses with an isolated two vessel umbilical cord. A review of eleven studies found a 4.4% stillbirth rate in otherwise normal fetuses with a two vessel umbilical cord – 10 times the rate in a control group with a three vessel umbilical cord. However, from these studies it was not clear as to whether the increased perinatal mortality with a two vessel umbilical cord was independent of intrauterine growth restriciton. The birth weight of twins with SUA is more than 500 g less than their co-twins.


The inner-to-inner umbilical artery diameter ranges from 1.2m m + 0.4 mm (mean + 1 SD) at 16-17 weeks’ gestation to 3.9 mm + 0.9 at 40-42 weeks’ gestation. With a single umbilical artery, the remaining artery is larger than normal and may approximate the diameter of the vein. The inner diameter of a SUA is increased by a factor of 1.25 compared with the diameter of an umbilical artery in a 3 vessel cord. Persutte and Lenke have suggested that a transverse umbilical artery diameter > 4 mm between 20-36 weeks’ gestation is predictive of a single umbilical artery. Since the diameter of the umbilical artery increases with gestational age, the use of a single umbilical artery diameter cut-off to detect a SUA will necessarily have a higher false negative rate earlier in gestation and a higher false positive rate in the latter part of the 3rd trimester. Sepulveda has proposed a gestational age independent ratio- where the internal diameter of the umbilical vein divided by internal diameter of the umbilical artery is less than 2 , to assist in the diagnosis of a SUA in the second and third trimesters. Color Doppler imaging permits an earlier diagnosis of a SUA than with traditional gray-scale. The intrafetal portion of the umbilical arteries pass on either side of the fetal bladder. By imaging the fetal pelvis in transverse section it can be determined, not only if there is a two vessel cord, but which vessel is absent. Abuhamad and co-workers, in a study of 77 fetuses with a SUA, reported that the left umbilical artery was more commonly absent than the right. They also found that cytogenetic and complex fetal anomalies only occurred in fetuses with an absent left artery. Two other reports involving 204 infants did not reveal any correlation between which umbilical artery was absent and the incidence of chromosomal or phenotypic anomalies. The common iliac artery on the side of the absent umbilical artery is of a smaller caliber than the common iliac artery on the side of a single umbilical artery. In addition, the distal aorta curves towards the common iliac artery on the side of the patent umbilical artery. An elevated S/D ratio in the umbilical artery has been used to identify those fetuses with a SUA who have the highest risk of an adverse outcome (i.e., growth restriction, increased perinatal mortality, complex anomalies and karyotypic malformations). A neonate with a two vessel umbilical cord has a 7-fold greater risk of having additional malformations than a neonate with a three vessel umbilical cord. The incidence of additional malformations with a SUA in neonates and in autopsy series is 21.1% and 69.4%, respectively. The identification of a single umbilical artery with a central nervous system anomaly suggests the presence of other anomalies and increases the likelihood of a karyotypic abnormality. Two congenital anomalies invariably associated with a SUA are an acardiac parabiotic twin and sirenomelia. In the latter malformation, the SUA is derived directly from the aorta. A single umbilical artery has been associated with karyotypic abnormalities, specifically trisomy 18 and trisomy 13. Nyberg et al have recommended that without additional anomalies, the prenatal detection of a SUA should not necessitate genetic amniocentesis. The author concurs with this recommendation, but with the following stipulations: 1) the sonologist performing the examination must be highly experienced; and 2) with a 2 vessel umbilical cord, a fetal anatomic survey should not be considered ”normal” until 20 weeks’ gestation. As gestational age advances, the detection rate of congenital anomalies associated with karyotypically abnormal fetuses increases. The latter stipulation permits the confirmation of a previously diagnosed 2 vessel umbilical cord, optimizes the detection rate of associated malformations, and still provides sufficient time to perform a genetic amniocentesis, if indicated. It must be emphasized that even with a second ultrasound examination between 20-22 weeks gestation, not all of the anomalies associated with a SUA will be detected. Persutte and Hobbins have estimated that only 32% of fetal anomalies associated with SUA will be identified during antenatal sonography.

Differential Diagnosis

The differential diagnosis of a single umbilical artery would include a single hypoplastic umbilical artery with a second umbilical artery of a normal diameter. In addition, the convergence of two umbilical arteries within the umbilical cord may lead to a transverse image of the cord with either two or three vessels. The later two abnormalities of the umbilical cord do not have the same prognostic significance as a two vessel umbilical cord.

Sonographic Features

A SUA has been associated with abnormalities of every organ system. The following list of the more common abnormalities associated with a two vessel umbilical cord derived from Persutte and Hobbins (1995) can be used as a guide in the further sonographic examination of an individual case.

Central Nervous System:




Cardiovascular System:

ventricular septal defect

tetralogy of Fallot

truncus arteriosis transposition hypoplastic left ventricle

Respiratory System:

pulmonary hypoplasia

diaphragmatic hernia

Gastrointestinal Tract:

duodenal atresia

tracheoesophageal fistula

Urinary Tract:

renal dysplasia

renal agensis


horseshoe kidney

Musculoskeletal System:

cleft lip and palate


club foot


Associated Syndromes

  • 45X Turner
  • Achondroplasia (1 case)
  • Anencephaly, iniencephaly, spina bifida
  • del 4p
  • Goltz
  • Maternal diabetes
  • Maternal epilepsy
  • Maternal hypertension
  • OEIS complex (exstrophy of cloaca)
  • Prune Belly
  • Sirenomelia/caudal agenesis
  • Triploidy
  • Trisomy 13
  • Trisomy 18
  • Trisomy 21
  • VATER association


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