Fetal age assessment based on femur length at 10–25 weeks of gestation, and reference ranges for femur length to head circumference ratios

Background.  The aim of the present study is to establish new reference charts for gestational age assessment based on fetal femur length (FL), and new reference ranges for FL to head ratios at gestational weeks 10–25, and to determine the effect of maternal and fetal factors on these charts. Methods. Six hundred fifty  low‐risk women with regular menstrual periods and singleton pregnancies were recruited to a prospective cross‐sectional study after obtaining written consent. FL, outer–outer biparietal diameter (BPD), and head circumference (HC) were measured at 10–25 weeks of gestation. We used regression analysis in order to construct mean curves and to assess the effect of maternal and fetal factors on age assessment. Results.  The new chart for age assessment by means of FL was based on 636 measurements. The 95% CI of the mean corresponded to <1 day. The variation between the mean and the 90th percentile was 5, 6, and 7 days at 13, 18, and 23 weeks, respectively, similar to the results when using BPD or HC. Maternal age modestly influenced gestational age assessment (1.3 days/10 years, P  = 0.005), whereas smoking, height, body mass index, multiparity, fetal sex, cephalic index, and breech presentation had no impact. Reference charts for FL to head ratios have been presented. Maternal age, fetal sex, and cephalic index influenced the FL/BPD ratio, whereas only fetal sex influenced FL/HC. Conclusions.  Fetal age assessment based on FL is an equally robust method as using HC. FL/HC is a more robust ratio to characterize fetal proportions than is FL/BPD.