Isolated Single Umbilical Artery and Fetal Karyotype

Ultrasound scans in the second and third trimesters of pregnancy have identified fetal anomalies related to chromosomal abnormalities in about 10% of fetuses with a single umbilical artery (SUA). Because fetal defects are identified in addition to the SUA in most chromosomally abnormal fetuses, it is unclear whether karyotyping is necessary for cases with isolated SUA. This study investigated the association between isolated SUA diagnosed during a routine second-trimester scan and chromosomal abnormalities, in order to determine whether fetal karyotyping is needed for pregnancies with an isolated SUA. All pregnant patients presenting for antenatal care and delivery at a fetal medicine unit between 2002 and 2008 were offered 2 ultrasound scans, the first at 11 to 13 weeks' gestation to screen for dysmorphisms associated with chromosomal defects and the second at 20 to 23 weeks for a detailed fetal examination. Another category of pregnant patients examined were referred from other hospitals because of suspected fetal abnormalities identified during their routine second trimester scan. Search of the fetal database identified 643 cases with SUA; of these, 424 (65.9%) were cases of isolated SUA, 133 (20.7%) were cases with 1 major defect, and 86 (13.4%) had multiple defects. Among these groups, the incidence of chromosomal abnormalities was 50.7% (37/73) in cases with multiple defects, 3.7% (4/108) in those with 1 defect, and 0% in those with isolated SUA. The most common chromosomal abnormalities in cases with multiple defects were trisomy 18, trisomy 13, and triploidy; these accounted for 91.9% (34/37) of cases. These findings suggest that fetuses with an isolated SUA are not at increased risk of chromosomal abnormalities. However, identification of an SUA in the second trimester requires careful examination for possible associated defects which do increase the risk of chromosomal abnormalities.