Objective: To determine whether current methods of detecting Down syndrome based on fetal femur length calculations are influenced by gestational age or maternal height. Methods: Four formulas were used to calculate expected femur length (FL) based on the fetal biparietal diameter (BPD) between 15 0/7 weeks’ gestation and 19 6/7 weeks’ gestation. For each gestational age, the BPD:FL ratio for women shorter than one standard deviation (SD) below the mean height was compared with the ratio for women taller than one SD above the mean height. A measured:expected FL ratio of 0.91 or less and a BPD:FL ratio greater than 1.5 SD above the mean was considered abnormal. Results: The four formulas used to calculate measured:expected FL ratios were significantly more likely to be abnormal at 15–16 weeks’ gestation, compared with 18–19 weeks’ gestation (P < .05). Maternal height correlated with femur lengths at 18 and 19 weeks’ gestation (P < .05) but not at earlier gestational ages. At 18 and 19 weeks’ gestation, women shorter than one SD below the mean were twice as likely to have an abnormal BPD:FL ratio compared with women taller than one SD above the mean (relative risk 2.38; 95% confidence interval 1.21, 4.69). Conclusion: Early gestational age increases a woman’s risk of having an abnormal measured:expected FL ratio, whereas short stature increases a woman’s risk of having an abnormal BPD:FL ratio at later gestational ages. These findings indicate that risk assessment for fetal Down syndrome for such patients might be inaccurate. (Obstet Gynecol 2001;97:742–6.