Objective: To examine the effects of method of conception on first trimester PAPP-A and free beta-hCG and the dependency of gestational age at the time of sampling on these effects Methods: Data on more than 100 000 spontaneously conceived and on 6 859 pregnancies conceived by assisted reproductive techniques (ART) with a first trimester risk assessment performed between 01 January 2008 and 31 October 2010 were retrieved from the Danish National Fetal Medicine Database. ART was defined as pregnancies obtained using in-vitro fertilization or intracytoplasmatic sperm injection. Multiple regression modelling was used to compare levels of PAPP-A and free beta-hCG in ART pregnancies. Results: After correction for maternal weight, ethnicity, smoking status and centre, levels of PAPP-A in ART pregnancies are significantly lower relative to spontaneously conceived pregnancies. This effect is significantly associated with gestational age. For every week increase in gestation, PAPP-A MoM values in ART pregnancies increase by an estimated 3%, from an estimated 0.85 MoM (95% CI 0.81 to 0.88) at week 9 to 0.93 MoM (95% CI 0.88 to 0.98) at week 12. Medians for free -hCG in pregnancies conceived by ART show a gestational age-specific increase relative to spontaneously conceived pregnancies. The weekly increase in free -hCG MoM levels is estimated to be 5%, increasing from 0.99 MoM (95% CI 0.95 to 1.04) at 9 weeks to 1.12 MoM at 12 weeks (95% CI 1.05 to 1.18). Conclusion: Adjustments for ART in first trimester screening for chromosomal abnormalities may need to be gestational age-specific.