Variation in fetal femur length with respect to maternal race

We sought to evaluate whether the expected fetal femur length, based on biparietal diameter, varies in second-trimester fetuses with respect to maternal race. The study population was composed of all fetuses scanned from 15 to 20 completed weeks' gestation during a 2-month period (June to August 1998). Maternal race was documented at the time of the ultrasonographic examination. 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 maternal race. The study subgroups were composed of the fetuses of 39 Asian mothers, 31 black mothers, and the first 100 white mothers. The mean values of the variance from the expected fetal femur length by biparietal diameter +/- 1 SD for the various racial groups were as follows: fetuses of Asian mothers, -0.66 +/- 1.64 mm; fetuses of black mothers, 0.88 +/- 1.57 mm; and fetuses of white mothers, 0.13 +/- 1.66 mm (P = .0007). To isolate the differences among the 3 racial groups, the mean values of the variance from the expected femur length by biparietal diameter for the fetuses of Asian and black mothers were compared with the mean value for the fetuses of white mothers (Asian versus white mothers, P = .014; black versus white mothers, P = .026). 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-expected femur lengths were noted among the fetuses of black mothers, compared 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 of various races require further study.