Isolated single umbilical artery and fetal karyotype.

OBJECTIVE To determine the need for fetal karyotyping in cases of an isolated single umbilical artery (SUA) identified during the second-trimester routine anomaly scan. METHODS All patients booked for antenatal care and delivery in our hospital are offered two ultrasound scans in pregnancy, one at 11-13 weeks' gestation as part of screening for chromosomal defects and another at 20-23 weeks for detailed fetal examination. In addition we examine patients referred from other hospitals because of suspected fetal abnormalities during their routine second-trimester scan. We performed a search of the database to retrieve all cases with an SUA and reviewed the ultrasound findings, fetal karyotype and pregnancy outcome. RESULTS There were 643 cases with SUA, including 424 (65.9%) where the condition was isolated, 133 (20.7%) with one major fetal defect and 86 (13.4%) with multiple defects. The incidence of chromosomal abnormalities was 0% in the isolated SUA group, 3.7% in those with one defect and 50.7% in those with multiple defects. The commonest chromosomal abnormalities were trisomy 18, trisomy 13 and triploidy, which together accounted for 82.9% of cases. CONCLUSION The finding of an SUA should prompt the sonographer to search for fetal defects and if these are found the risk for chromosomal abnormalities is increased. In cases of apparently isolated SUA there is no evidence of increased risk of chromosomal abnormalities.