Objective To determine the possible association between single umbilical artery (SUA) at 11–14 weeks of gestation and the incidence of chromosomal abnormalities. Methods Color flow imaging of the fetal pelvis was used to determine the number of umbilical arteries in 717 fetuses immediately before chorionic villus sampling for karyotyping at 11–14 weeks ’ gestation. Results Single umbilical artery (SUA) was diagnosed in 21/634 (3.3%) chromosomally normal fetuses, in 5/44 (11.4%) with trisomy 21, 14/18 (77.8%) with trisomy 18 and 2/21 (9.5%) with other chromosomal defects. In the chromosomally normal group there was no significant difference in median fetal crown–rump length or nuchal translucency (NT) between those with a single and those with two umbilical arteries. In the 42 fetuses with SUA the expected number of cases of trisomy 21, estimated on the basis of maternal age, gestational age and fetal NT, was 4.7, which was not significantly different from the observed 5. The corresponding numbers for trisomy 18 were 2.0 for expected and 14 for observed (Fisher’s exact test P = 0.0016). Conclusion A SUA at 11–14 weeks ’ gestation has a high association with trisomy 18 and other chromosoma
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