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

Obstet Gynecol. 2001 May;97(5 Pt 1):742-6. doi: 10.1016/s0029-7844(01)01319-9.


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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Body Height*
  • Cohort Studies
  • Confidence Intervals
  • Down Syndrome / diagnostic imaging*
  • Down Syndrome / epidemiology
  • Embryonic and Fetal Development
  • Female
  • Femur / embryology*
  • Femur / growth & development*
  • Gestational Age*
  • Humans
  • Predictive Value of Tests
  • Pregnancy
  • Prevalence
  • Probability
  • Risk Assessment
  • Sensitivity and Specificity
  • Ultrasonography, Prenatal / methods*