This retrospective study compared fecundity of couples according to the contraceptive method used before pregnancy, taking into account various potential confounding factors, particularly the waiting period adopted by some women after stopping contraception during which another method was used. 3398 pregnant women who had discontinued contraception in order to achieve a desired pregnancy completed questionnaires between June 1980-June 1982. The mean age of the women at last menstruation was 26.3 years. 37.6% were primaparas, 87.3% were employed, and 36.6% smoked. 2437 had used oral contraceptives (OCs), 492 IUDs, and 469 other methods. 60.4% of pill users, 29.7% of IUD users, and 6.1% of users of other methods had observed waiting periods averaging 3.8, 2.7, and 2.5 months respectively. The delay necessary to conceive (DNC) averaged 3.4 months and the proportion conceiving during their 1st cycle of unprotected intercourse was 36%. The DNC was shorter and the percent conceiving in their 1st unprotected cycle was higher in couples observing a waiting period, in couples not following a strategy to increase the probability of conception by concentrating intercourse in midcycle, and in women with regular cycles. The DNC was not significantly related to maternal age, but the proportion conceiving in the 1st unprotected cycle was 28.0% for women aged 15-19, 40.8% for women aged 25-29, and 32.6% for those over 35. The 4 factors were also closely related to the contraceptive method used. OC users more often observed a waiting period, less often observed an intercourse timing strategy, more frequently had irregular cycles, and were younger than users of IUDs or other methods. A comparison of couples observing or not observing a waiting period, with use of a strategy, regularity of cycles, and age held constant, indicated a significant decline in fecundity among women observing no waiting period after discontinuing the pill. Among women not observing a waiting period, 21% of pill users, 30% of IUD users, and 43% of other method users became pregnant in the 1st unprotected cycle, and the DNC in months was 5.0 for pill users, 3.4 for IUD users, and 3.1 for other method users. Fecundity seemed to be unaffected in couples observing a waiting period regardless of the type of contraception previously used: 45% discontinuing pill use, 46% discontinuing IUD use, and 51% discontinuing other methods became pregnant in the 1st unprotected cycle. Reduction of fecundity in couples discontinuing pill use probably reflects the high proportion of anovulatory or dysmenorrheic cycles immediately after pill use, while the decline in fecundity after IUD use may be explained by abrasion of the vaginal mucosa when the device is withdrawn.