A short femur roughly measures at least one week behind all other biometric parameters in the second trimester. In the medical literature, a short femur is technically defined as being present when the actual femur length is < 0.91 times the expected value, or the biparietal diameter/femur length ration is > 1.5 SD above the mean. The data regarding the efficacy of femur length ratios for the detection of Down syndrome differs significantly among studies, and there is considerable overlap between affected and control pregnancies.
Given the inconsistency of the data regarding femur length as a predictor for Down syndrome, attempts have been made to use other measurement ratios, including humerus length, humerus plus femur length, humerus and/or femur length versus foot length, again with conflicting results.
The variation among studies may be attributed to differences in ultrasound equipment and technique between centers, differences in gestational ages of pregnancies studied, the tendency to disregard femur length or biparietal diameter measurements that appear inconsistent with the gestational age, or racial variability of the population served.
The finding of short femur and/or short humerus should prompt a search for additional markers for Down syndrome. Given the isolated finding of short femur and/or humerus, we recommend offering genetic counseling.
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