Comparison of ultrasound femur length and biparietal diameter in late pregnancy.

A linear relationship between growth of fetal femur length (FL) and biparietal diameter (BPD) after 22 weeks' gestation is described. The normal ratio of femur length to BPD (FL/BPD ratio) was found to be 79 +/- 8%. Effective uses of the FL/BPD ratio include its use as a quality control check on femur length and BPD measurements and its use to diagnose short-limbed dwarfism, hydrocephalus, and microcephaly. Pregnancy dating by means of BPD can now be cross checked by use of femur measurements and new observations on normal and abnormal fetal growth will now be possible.