Influence of gestational age and maternal height on fetal femur length calculations

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) ba sed 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 c ompared 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 tha n 1.5 SD above the mean was considered abnormal. Results: The four formulas used to calculate measured:expected FL ratios we re significantly more likely to be abnormal at 15-16 weeks' gestation, comp ared 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 ges tational ages. At 18 and 19 weeks' gestation, women shorter than one SD bel ow 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% confi dence interval 1.21, 4.69), Conclusion: Early gestational age increases a woman's risk of having an abn ormal measured:expected FL ratio, whereas short stature increases a woman's risk of having an abnormal BPD:FL ratio at later gestational ages. These f indings indicate that risk assessment for fetal Down syndrome for such pati ents might be inaccurate.