Fetal body ratios in second trimester: a useful tool for identifying chromosomal abnormalities?

This study was carried out to determine if second trimester fetal body ratios are useful in detecting chromosomally abnormal fetuses. As a reference population, normative data for five fetal body ratios (femur length/biparietal diameter, biparietal diameter/fetal length, femur length/head circumference, head circumference/abdominal circumference, and femur length/abdominal circumference) were derived using regression analysis from a population of chromosomally normal fetuses (n = 1770) who underwent genetic amniocentesis at our institution between 14 and 21 menstrual weeks. During the same time period, 37 chromosomally abnormal fetuses were identified by amniocentesis. In comparing the two groups using the 10th and 90th percentiles as cutoffs between normal and abnormal, approximately 25% of chromosomally abnormal fetuses were identified, whereas approximately 20% of the normal fetuses were incorrectly classified as abnormal. Moreover, the use of 1.5 standard deviations above the mean for BPD/FL identified only 19% of Down syndrome fetuses. Our data, and those from a comprehensive review of the literature, suggest that the sensitivity of these ratios in detecting chromosomally abnormal fetuses is too low to recommend them for routine screening.