Down syndrome is one of the most important causes of mental retardation in the population. In the absence of prenatal screening and diagnosis, prevalence at birth in the United States would currently exceed 1: 600. the purpose of prenatal screening is to identify those women at the increased risk for an affected pregnancy and to maximize the options available to these women. Tests available: second trimester serum screening involves combing the maternal age- specific risk for an affected pregnancy with the risks associated with the concentrations of maternal serum alpha- fetoprotein (MSAFP), unconjugated estriol (uE3), and human chorionic gonadotrophin (hCG) (triple testing). A forth analyte, inhibin-A, (Inh-A), is increasly being utilize (quadruple testing). Optimal second trimester screening requires the integration of a number of clinical variables, the most important of which is an accurate assessment of gestational age. In addition to Down Syndrome, the triple and Quadruple tests preferentially identify fetal trisomy 18, Turner syndrome, Triploidy, Trisomy 16 mosaicism, fetal death, Smith- Lemli- Opitz Syndrome, and steroid sulfatease deficiency.