Variation in fetal femur length with respect to maternal race

We sought to evaluate whether the expected fetal femur length, based on bip arietal diameter, varies in second-trimester fetuses with respect to matern al race. The study population was composed of all fetuses scanned from 15 t o 20 completed weeks' gestation during a 2-month period (June to August 199 8). Maternal race was documented at the time of the ultrasonographic: exami nation. Biparietal diameter and femur length were prospectively documented. The variance from the expected femur length, given the biparietal diameter , was calculated, and the mean variations were compared according to matern al race. The study subgroups were composed of the fetuses of 39 Asian mothe rs, 31 black mothers, and the first 100 white mothers. The mean values of t he variance from the expected fetal femur length by biparietal diameter +/- 1 SD for the various racial groups were as follows: fetuses of Asian mothe rs, -0.66 +/- 1.64 mm; fetuses of black mothers, 0.88 +/- 1.57 mm; and fetu ses of white mothers, 0.13 +/- 1.66 mm (P = .0007). To isolate the differen ces among the 3 racial groups, the mean values of the variance from the exp ected femur length by biparietal diameter for the fetuses of Asian and blac k mothers were compared with the mean value for the fetuses of white mother s (Asian versus white mothers, P = .014; black versus white mothers, P = .0 26). A significant difference in the mean variance from the expected femur length by biparietal diameter was identified among the fetuses of women in the second trimester with respect to racial group. Less-than-expected femur lengths were noted among the fetuses of Asian mothers, and greater-than-ex pected femur lengths were noted among the fetuses of black mothers, compare d with the femurs of fetuses of white mothers. The implications for the use of fetal femur length as a component of the genetic sonogram in patients o f various races require further study.