RESUMO
OBJECTIVE: We investigated the usefulness of shortening of the fetal femur length (FL) to predict Down syndrome at different gestational ages in Korean subjects. METHODS: This study involved 110 Korean Down syndrome fetuses and 602 randomly selected euploid controls. The expected FL for any biparietal diameter (BPD) was calculated based on the control group data. Subjects were divided into four groups according to gestational age periods: 14-18 weeks; 19-23 weeks; 24-28 weeks and 29-36 weeks. The value of measured/expected FL ratio to predict Down syndrome was analyzed for each group. RESULTS: The values of FL for any BPD in Down syndrome patients were significantly different from those in the control group (p < 0.001). A low ratio of measured/expected FL increased the risk of fetal Down syndrome (p < 0.001) with a mean measured/expected FL in Down syndrome of 0.907 (SD 0.075). At a fixed false positive rate of 5%, the sensitivities of FL were lower than 32.8% (95% CI 0.705-0.915) in three second trimester groups, and 71.1% (95% CI 0.924-0.997) in the third trimester group. CONCLUSION: Short FL is a poor marker of Down syndrome in the second trimester in Korean subject. It may be used as a screening marker only in the third trimester.