Assessment of gestational age in the second trimester by real-time ultrasound measurement of the femur length.

Proposed is a new method for the prediction of gestational age in the early second trimester by ultrasound measurement of the femur length. A growth curve of the calcified section of femur was constructed which indicated that gestational age could be predicted with 95% confidence limits to +/- 6.7 days. This hypothesis was tested on 47 "blind" studies and found to be reliable. The femur length measurements proved to be reproducible with a mean standard deviation in 30 experiments of 0.8 mm and compared with a radiologic model. This adds a new parameter for estimating fetal age by ultrasound which would appear to be as accurate as that of biparietal diameter. Although this is not intended to replace the present parameters, it may be useful as an additional measurement to ensure the accuracy of estimated fetal age.