Down syndrome is known as one of the most common causes of mental retardation. Screening for Down's syndrome in the second trimester of pregnancy, based on the concentrations of various markers in serum and maternal age, has become widely used in the past decade. Down's syndrome is associated with low maternal serum alpha-fetoprotein and unconjugated estriol concentrations and high maternal serum human chorionic gonadotropin and inhibin A concentrations. Measurements of the first three markers, in addition to age, constitute the widely used triple test; measurements of all four markers with age make up the quadruple test. The triple test shows 62% of detection rate and the quadruple test was substantially better than the triple test (70%). Also several sonographic markers, especially nuchal translucency and nasal bone apprears to be a highly effective method to screen for Down syndrome.