Objective: To determine if gestational age at serum sampling affect the discriminative value of PAPP-A and free β hCG in relation to trisomy 18 (T18) and trisomy 13 (T13). Methods: We retrospectively searched for T18 and T13 cases in the Astraia database at two large Fetal Medicne Centres in Denmark (Copenhagen University Hospital, Rigshospitalet, and Aarhus University Hospital, Skejby). First trimester screening data was retrieved, including cytogenetic and biochemical results (PAPP-A and free β hCG). Regression analysis was applied to model the relationship between log MoM values of PAPP-A and free β hCG. We also reviewed prospective data from the centres above as well from Copenhagen University Hospital, Hvidovre, with repeated biochemical sampling: An with early BC sample taken prior to the NT scan, and a late BC sample taken at the time of the NT scan. Results: In the retrospective data, 39 pregnancies with trisomy 18 and 26 pregnancies with trisomy 13 was identified. We found that PAPP-A MoM levels in trisomy 18 pregnancies are less discriminatory (P = 0.0004) at earlier gestations than they are at later gestations. They decrease from an estimated median MoM of 0.54 (95% CI: 0.41 to 0.71) at the middle of week 9 to 0.22 (95%CI: 0.16 to 0.31) at the middle of week 12. Levels of free β hCG also showed a trend towards poorer discriminatory power at earlier gestations (P = 0.08) decreasing from a median MoM of 0.32 (95% CI: 0.26 to 0.39) at the middle of week 9 to 0.22 (95% CI: 0.16 to 0.30) at the middle of week 12. For trisomy 13 pregnancies we also found a trend, though not statistically significant, towards poorer discrimination at early gestations. In the prospective, two sample data, a total of 5 T18 and 3 T13 cases was identified. The within-case MoM-variation showed a trend toward lower MoM-values with higher gestational age for both T18 and T13 cases. Due to low numbers, the difference didn’t reach statistical significance. Conclusions: First trimester screening for trisomies 18 and 13 is more effective using maternal PAPP-A and free β hCG serum samples taken at later gestations than at earlier gestations. Risk assessments may need to take account of the changing discriminatory pattern with gestational age.