Objective: Investigate gestational age dependent effects of racial origin, smoking status and mode of conception on maternal serum levels of free ß-human chorionic gonadotropin (β-hCG) and pregnancy associated plasma protein-A (PAPP-A) at 7-14 weeks' gestation. Methods: Data arise from prospective first-trimester combined screening for aneuploidies in singleton pregnancies, with serum free ß-hCG and PAPP-A measured at 7(+1) -14(+3) weeks' gestation. We include 27,908 pregnancies from three centres in the UK and 125,461 pregnancies from 22 centres in Denmark with normal fetal karyotype or birth of a phenotypically normal neonate. Multiple regression modelling of log(10) -transformed marker concentrations was used to produce log(10) multiple of the median (MoM) values for free β-hCG and PAPP-A and to examine pregnancy characteristics that have significant effects on marker concentrations. Results: Serum free ß-hCG and PAPP-A concentrations were significantly affected by gestational age, maternal weight, racial origin, parity, smoking and mode of conception. There were significant gestational age dependent effects of Afro-Caribbean race, smoking and IVF conception on PAPP-A and free ß-hCG levels. In women of Afro-Caribbean race there was a weekly increase in PAPP-A of 5.3% and in free ß-hCG of 1.8%. In smokers there was a weekly decrease in PAPP-A of 2.4% and in free β-hCG of 1.6%. In IVF conceptions there was a weekly increase in PAPP-A of 4.5% and in free ß-hCG of 4.6%. Conclusions: Serum free ß-hCG and PAPP-A concentrations at 7-14 weeks are affected by several pregnancy characteristics. The effects of Afro-Caribbean race, smoking and IVF conception change with gestational age. Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.