Use of femur length measured by ultrasonography to predict fetal maturity.

The ultrasonographic criteria of biparietal diameter greater than or equal to 9.2 cm and a grade III placenta have been shown to correlate with fetal maturity in the fetus of a nondiabetic mother in our population. These two criteria have helped reduce the incidence of third-trimester amniocenteses by 66.6% at our institution. Occasionally, however, because of fetal position, we are unable to obtain an accurate biparietal diameter. We therefore carried out a prospective study of 120 patients to evaluate the relationship between femur length of greater than or equal to 7.3 cm and fetal maturity. In this study, the addition of the femur length criterion increased the specificity of the ultrasonographic criteria for fetal maturity from 66.6% to 85%. The data suggest that femur length could be used along with the biparietal diameter and placental grade as an alternative to amniocentesis in term nondiabetic pregnancies.