Objective: This time to pregnancy study was carried out to find the likelihood of spontaneous conception in subsequent cycles as this is of importance for a balanced management of infertility. Previous studies on time to pregnancy are only retrospective and biased because of exclusion of truly infertile couples. Additionally, age-related effects on fecundity are only analyzed appropriately using cumulative probabilities of conception (CPC).Design: Prospective observational study on users of natural family planning methods trying to achieve a pregnancy.Material and Methods: We observed 346 unselected women who used natural family planning methods to conceive from their first cycle onward. The CPC were estimated for the unselected group and for couples who finally conceived calculating Kaplan-Meier survival rates. Influencing factors, especially age, were analyzed as well.Results: Three hundred ten pregnancies occurred among 346 women. The pregnancy rates at 1, 3, 6, and 12 cycle(s) were 38%, 68%, 81%, and 92%. For those who finally conceived, the respective pregnancy rates were 42%, 75%, 88%, and 98%. For these pregnant women, CPC were statistically age independent and the final cumulative pregnancy rates were not different. For all, CPC decreased with age.Conclusions: Most couples conceive quite early. The rest is probably sub- or infertile and should be investigated for infertility problems after six unsuccessful cycles with fertility-focused intercourse to avoid infertility under- and over-treatment. Couples with unexplained infertility may be advised to wait as they have a reasonable good chance to conceive spontaneously. The others may soon switch to further diagnosis and treatment, regardless of age. With age, CPC decline because heterogeneity in fecundity increases due to a higher proportion of infertile couples. In truly fertile couples, CPC are probably age independent. Thus, the fertility of a woman may not decline gradually with age. Objective: This time to pregnancy study was carried out to find the likelihood of spontaneous conception in subsequent cycles as this is of importance for a balanced management of infertility. Previous studies on time to pregnancy are only retrospective and biased because of exclusion of truly infertile couples. Additionally, age-related effects on fecundity are only analyzed appropriately using cumulative probabilities of conception (CPC). Design: Prospective observational study on users of natural family planning methods trying to achieve a pregnancy. Material and Methods: We observed 346 unselected women who used natural family planning methods to conceive from their first cycle onward. The CPC were estimated for the unselected group and for couples who finally conceived calculating Kaplan-Meier survival rates. Influencing factors, especially age, were analyzed as well. Results: Three hundred ten pregnancies occurred among 346 women. The pregnancy rates at 1, 3, 6, and 12 cycle(s) were 38%, 68%, 81%, and 92%. For those who finally conceived, the respective pregnancy rates were 42%, 75%, 88%, and 98%. For these pregnant women, CPC were statistically age independent and the final cumulative pregnancy rates were not different. For all, CPC decreased with age. Conclusions: Most couples conceive quite early. The rest is probably sub- or infertile and should be investigated for infertility problems after six unsuccessful cycles with fertility-focused intercourse to avoid infertility under- and over-treatment. Couples with unexplained infertility may be advised to wait as they have a reasonable good chance to conceive spontaneously. The others may soon switch to further diagnosis and treatment, regardless of age. With age, CPC decline because heterogeneity in fecundity increases due to a higher proportion of infertile couples. In truly fertile couples, CPC are probably age independent. Thus, the fertility of a woman may not decline gradually with age.