Evaluation of fetal femur length to detect Down syndrome in a Thai population

Objective To assess the value of femur length shortening for prenatal detec tion of Down syndrome in a Thai population. Method: A prospective study was performed by experienced perinatologists on 3137 women undergoing second-t rimester amniocentesis, between 16 and 24 weeks of gestation, for the indic ations of advanced maternal age and past history of chromosomal abnormality . Biparietal diameter and femur length measurements were obtained before th e procedures. Regression equations relating biparietal diameter to femur le ngth were used to calculate observed femur length/expected femur length rat io in the chromosomally normal and Down syndrome fetuses. Sensitivity, spec ificity, false-positive rate and likelihood ratio of a positive test result at various observed femur length/expected femur length ratios for detectio n of Down syndrome were calculated. A receiver-operator characteristic curv e was used to determine threshold screening ratio. Results: There were 3084 chromosomally normal pregnancies, 26 fetuses with Down syndrome (1:118), a nd 27 other chromosomal abnormalities. The relationship between femur lengt h and biparietal diameter (BPD) was: expected femur length - -7.631 + 0.814 BPD, R-2 = 0.78, P < 0.001). Femur length in Down syndrome fetuses was sig nificantly shorter than in normal fetuses (P<0.001). A ratio of 0.91 for ob served femur length/expected femur length yielded a sensitivity of 42.3%, s pecificity of 86.2%, false positive rate of 13.8% and likelihood ratio of a positive test result of 3.07 (95% CI 1.94-4.84) for detection of Down synd rome. Conclusions: In this study, femur length shortening in the second tri mester appears to be a useful screening parameter for fetal Down syndrome i n a Thai population. (C) 2001 International Federation of Obstetrics and Gy necology. All rights reserved.