Relation of fetal femur length to neonatal crown-heel length

Sonographically measured femur lengths in 102 fetuses were compared with neonatal crown-heel lengths within 72 hours of delivery. The optimal model for describing crown-heel length as a function of femur length was a linear model, with a coefficient of determination (r2) of 66 per cent; the standard deviation in predicting crown-heel length from femur length was 2.4 cm. These data were compared with autopsy data from the literature, which showed an even stronger correlation between femur length and crown-heel length. Because of the strong correlation between these two dimensions, the femur length should prove useful in the detection of intrauterine growth retardation when crown-heel length is compromised. It should also be useful as a variable in the prediction of fetal weight in utero, since crown-heel lengths at the extremes of normal can significantly affect in utero weight estimates using ultrasound.