Abstract: A prenatal screening test for risk of Down's Syndrome in a foetus, which is carried out on a maternal blood sample before the beginning of the third trimester of pregnancy, by assaying the sample for a metabolite of feto-placental activity which is affected by the presence of Down's Syndrome, selected from unconjugated oestriol; progesterone; 16-alpha-hydroxy-dehydroepiandrosterone sulphate (16-alpha-hydroxy-DHEAS); and dehydroepiandrosterone sulphate (DHEAS). An additional assay for alpha-fetroprotein and/or human chorionic gonadotrophin may be included.