BACKGROUND: All conceptions occurring after a treatment for infertility are usually attributed to the treatment. This is only true when only totally sterile couples are treated: subfecund couples may still conceive without treatment. METHODS: We used a computer simulation (Monte Carlo) model of reproduction, combining the monthly probability of conceiving, the risk of miscarriage and the probability of age dependent permanent sterility. After a delay (D) without conceiving, the couples are assumed to turn to assisted reproduction technology. The treatment increases the fecundability (monthly rate of conception) of subfecund couples and restores the fertility of some totally sterile couples. The overall effectiveness of the therapy is based on data (by age of women) for two attempts of IVF. The number of conceptions ending in a live birth with ART is compared to the number of conceptions after one or two additional years without ART. RESULTS: At 30 years of age, the crude effectiveness of ART, based on all conceptions after treatment, is double that observed without treatment (30% vs. 15%), and beyond 41 years of age, the number of conceptions with treatment is no higher than without treatment over two years of exposure to conception (15% for both at 41 years of age, 7% at 45 years of age). Shortening the delay (D) does not improve the total number of women conceiving and having a live birth unless we assume the couple try again to spontaneously conceive. CONCLUSIONS: The chances of conceiving spontaneously must not be ignored when defining an infertility treatment, especially in cases of unexplained infertility. Shortening the delay before using ART is not in itself a more efficient strategy.