The utility of fetal biometry as an adjunct to the multiple-marker screening test for Down syndrome

Our purpose was to investigate fetal biometry as an adjunct to the multiple-marker screen (maternal age, serum alpha-fetoprotein, estriol, and human chorionic gonadotropin) for Down syndrome.

Fifty-two cases of Down syndrome were compared with 7514 normal fetuses. The measured/predicted femur length ratio had the best discriminant value (1.0 +/- 0.11 vs 0.93 +/- 0.13, p < 0.0001). Multivariate gaussian algorithms were developed and each computed a likelihood ratio for Down syndrome. The trivariate algorithm incorporated the three maternal analytes, whereas the quadrivariate version also included the femur length ratio. The study population included 38 cases of Down syndrome and 1098 euploid controls. The midtrimester risk was the product of the age-related risk and the likelihood ratio.

The relative difference in the femur length ratio between normal and affected fetuses was small in comparison to that of the maternal serum analytes. At a risk cutoff of > or = 1:190 the detection rates were similar and actually favored the trivariate algorithm but differed only by one case of Down syndrome.

The addition of the measured/predicted femur length ratio had a negligible effect on the performance of the multiple-marker screening test.