BACKGROUND Over the past decades, the proportion of couples who resort to infertility treatment has tremendously increased, and fertility (the final number of children) sharply declined. We explored the roles of two potential causes of these trends: a temporal decline in the couples' fecundability and a postponement of age at initiation of pregnancy attempts. METHODS We conducted a Monte-Carlo simulation for the reproductive history of 100 000 women based on the fertility and socio-demographic characteristics of the 1968 birth cohort in France. Declines in fecundability of various amplitudes have been implemented, as well as increases in the distribution of age at initiation of pregnancy attempts. RESULTS A decline in fecundability by 15% implied a decrease in fertility by 4%, and an increase in the proportion of couples eligible for infertility treatments by 73%. An increase in the mean age at initiation of first pregnancy attempt by 2.5 years from 25 years entailed a decrease by 5% in fertility and an increase by 32% in the proportion of couples eligible for infertility treatments. CONCLUSION A relatively important decrease in fecundability and an increase by 2.5 years in age at first pregnancy attempt are likely to have only a limited impact on fertility. However, they may have a large impact on the proportion of involuntarily infertile couples, likely to resort to assisted reproduction techniques.