AFP and Down syndrome: Down syndrome (DS) screening is the routine prenatal biochemical screening, which can calculate the risk of fetus trisomy 21 (T21), trisomy 18 (T18) and open neural tube defect by detecting the levels of free β-subunit of human chorionic gonadotropin in maternal serum (fβhCG), alpha fetoprotein (AFP) and unconjugated estriol (uE3) combined with other information about the pregnant women5,6.