This study evaluates the effectiveness of using human chorionic gonadotrophin (hCG) concentrations in maternal serum during the second trimester to improve the detection of Down's syndrome. The median hCG concentration in 77 pregnancies associated with Down's syndrome was twice that in 385 unaffected pregnancies, matched for maternal age, gestational age, and serum storage duration. Measuring hCG in maternal serum was found to be an effective screening test, with a lower false positive rate (3%) compared to maternal age (5%) and existing serum screening tests (7% for unconjugated oestril and 11% for α-fetoprotein). Combining hCG with other variables yielded the best results, achieving a detection rate of 60% with a false positive rate of 5%. This new screening method could reduce the number of babies born with Down's syndrome in the United Kingdom from about 900 to about 350 annually. The study also suggests that maternal age should no longer be the primary screening variable but should be used in combination with other tests for all pregnant women.This study evaluates the effectiveness of using human chorionic gonadotrophin (hCG) concentrations in maternal serum during the second trimester to improve the detection of Down's syndrome. The median hCG concentration in 77 pregnancies associated with Down's syndrome was twice that in 385 unaffected pregnancies, matched for maternal age, gestational age, and serum storage duration. Measuring hCG in maternal serum was found to be an effective screening test, with a lower false positive rate (3%) compared to maternal age (5%) and existing serum screening tests (7% for unconjugated oestril and 11% for α-fetoprotein). Combining hCG with other variables yielded the best results, achieving a detection rate of 60% with a false positive rate of 5%. This new screening method could reduce the number of babies born with Down's syndrome in the United Kingdom from about 900 to about 350 annually. The study also suggests that maternal age should no longer be the primary screening variable but should be used in combination with other tests for all pregnant women.