Fetal femur length as a predictor of menstrual age: sonographically measured.

The relation between fetal femur length and menstrual age was determined by cross-sectional analysis of 338 normal fetuses (12--40 weeks) using real-time sonography. Mathematical modeling of the data demonstrated that the femur growth curve is nonlinear, similar to the biparietal diameter growth curve. Predicted femur length values at various points in gestation were comparable to the results of other investigators. Predicted menstrual age values in weeks for specific femur length measurements in millimeters were calculated and are reported in tabular form. The variability (+/- 2 SD) associated with predicting menstrual age from femur length is +/- 9.5 days between 12--23 weeks, but between 23--40 weeks the variability is +/- 22 days. Fetal femur length can be used as an adjunct in estimating menstrual age, and as a screening device for diseases that affect fetal limb growth (e.g., dwarfism, osteogenesis imperfecta).