<prompt> Biochemical analytes of combined test of first-trimester screening isAFP and b-hcgb-hcg and PAPP-AAFP and PAPP-AInhibin and PAPP-A  <response>: 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: Williams Obstetrics; 24th edition; Chapter 14; Prenatal diagnosis 