Quadruple screening is ideally performed between 15 and 18 weeks of gestation but can extend to 22 weeks.3 Studies indicate that sequential screening in both the first and second trimesters detects more Down syndrome cases.4 Aberrations in maternal serum markers, such as decreased alpha-fetoprotein and estriol levels alongside elevated hCG, are associated with Down's syndrome.5 This evidence concerns the gene AFP and Down syndrome.