A fetoprotein assay is performed by drawing a vein of blood from the pregnant woman. A fetoprotein measurement in pregnant women during pregnancy is a screening test for Down’s syndrome. The alpha-fetoprotein in pregnant women during pregnancy is actually produced by the liver cells of the fetus. Typically starting in the fourth month of pregnancy, fetal liver cells produce relatively high levels of alpha-fetoprotein and release some of it into the mother’s circulation, and the concentration of alpha-fetoprotein in the mother’s blood rises significantly. In the middle of the pregnancy, i.e. around 16-18 weeks, the chances of the fetus having Down’s syndrome are assessed by measuring the concentration of alpha-fetoprotein in the mother’s blood, the concentration of free estriol, and the concentration of HCG, and by making a comprehensive analysis and judgment in conjunction with the mother’s age, weight, and week of gestation.