Potential prenatal predictions of Down syndrome: a statistical analysis.

To determine the feasibility of combining several screening tests for the prenatal detection of Down syndrome, we evaluated the potential relationship among three proposed predictors. We determined the concentration of chorionic gonadotropin in frozen serum samples from women of known maternal age and weight, fetal biparietal diameter, and femur length, and alpha-fetoprotein concentration. When corrected for gestational age and maternal weight, the potential predictors were independent, except for a slight correlation (r = 0.10) between maternal serum alpha-fetoprotein and maternal serum human chorionic gonadotropin. Both maternal serum human chorionic gonadotropin and biparietal diameter/femur length demonstrated an approximately log-normal distribution similar to maternal serum alpha-fetoprotein. Therefore it is scientifically sound to use any or all of these variables in combination for the identification of pregnancies at increased risk for Down syndrome.