single umbilical artery

diagnosis

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.