Invited commentary: the potential for monitoring of fecundity and the remaining challenges.

In the epidemiologic study of reproductive capacity, the assessment of fecundity as a functional measure is complementary to approaches that focus on biomedical mechanisms and/or that use biomarkers such as semen quality. More research is needed on time trends, spatial patterns, and particular groups, especially those exposed to potentially toxic agents. Although specific research projects will always be important, much could be gained by general population surveillance, which could be introduced into existing multipurpose surveys and repeated periodically. The core measurement would be time to pregnancy, which can be carried out using a short, acceptable questionnaire that has good validity at the group level. This should be accompanied by questions on time periods of unprotected intercourse that do not end with conception, to avoid bias resulting from exclusion of relatively infertile couples. Information is also required on contraceptive failures, recent contraceptive use, and other covariates and possibly on behavioral variables, such as the degree of planning and persistence in trying to conceive, and couples' knowledge of fertile days of the menstrual cycle. Existing statistical methods can deal with possible biases due to "accidental" pregnancies and the effects of fertility treatment. Further methodological work is needed to avoid or measure more subtle biases, for example, to determine the best way to deal with pregnancies occurring to couples whose approach to family formation is relaxed, for whom the concept of "pregnancy planning" does not apply.