The best time for Down’s syndrome screening is between 15-20+6 weeks of gestation. The test is performed by taking blood from a pregnant woman’s vein to measure the concentration of HCG, alpha-fetoprotein and free estriol in her blood, which is combined with her age and weight into a formula to calculate the risk of Down’s syndrome in the fetus. Down syndrome is a fetal chromosomal abnormality that is characterized by congenital mental retardation combined with multiple organ and fetal structural malformations, resulting in a fetus with a low life expectancy after birth.