Fetal femur length to detect trisomy 21. A reappraisal

An association between short femur length (FL) relative to the biparietal diameter (BPD) measured in the second trimester and the Down syndrome recently has been reported by Benacerraf et al (1987). Fetuses meeting an easily calculated criterion were stated to have a high probability for trisomy 21. We compared the biometric data from a selected group of 229 normal second trimester fetuses with 30 fetuses of similar gestational age with karyotype-proven trisomy 21. We found a substantial difference in femur lengths of normal fetuses in our population compared to that reported by Benacerraf et al. The reason for the difference is unclear. Further, we were unable to demonstrate a statistically discernible deviation of measured versus predicted femur lengths in the Down syndrome cohort employing either the formula recommended by Benacerraf et al or a formula calculated from our normal cohort.