Objective: To prospectively evaluate the screening performance of first trimester combined screening for trisomy 21 using a double set of biochemical markers Methods: Three fetal medicine departments in Denmark participated in the study. Screening for trisomy 21 was set up as a two-step approach with an early blood sample taken prior to the NT scan, and another blood sample taken at the time of the NT scan. PAPP-A and free β-hCG were measured on both the early and the late samples, and Multiples of the Median (MoM) values were calculated in addition to the corresponding trisomy 21 risk. Using statistical modelling we estimated detection rates (DR) and false positive rates (FPR) when using early sampling, late sampling or combinations of early and late sampling. Results: We collected two blood samples in 25 pregnancies affected by trisomy 21 and in 3942 control pregnancies. The early samples were taken between gestational week 8+0 to 13+6, and the late samples were taken between week 11+3 and 14+6. The median interval between the samples was 17 days (range 1-40 days). The best performance of the combined screening was obtained using PAPP-A from the early sample and free β-hCG from the late sample, with a 95% DR for a 2.5 % FPR at a cut off 1:100. Conclusion: Using repeated biochemical sampling in the first trimester with early PAPP-A and late free β-hCG can optimize the screening performance of combined screening for trisomy 21.