Biochemical analytes of combined test of first-trimester screening is

Two analytes used for first-trimester aneuploidy screening are human chorionic gonadotropin and pregnancy-associated plasma protein A (PAPP-A). In cases of fetal Down syndrome , the first-trimester serum free b-hcg level is higher, approximately 2.0 MoM, and the PAPP-A level is lower, approximately 0.5 MoM. With trisomy 18 and trisomy 13, levels of both analytes are lower.Reference: William's Obstetrics; 24th edition; Chapter 14; Prenatal diagnosis