A follow-up study of environmental and biologic determinants of fertility among 430 Danish first-pregnancy planners: design and methods

This paper presents design and methods of a follow-up study of fecundability (defined as the probability of conception during one menstrual cycle) in first-pregnancy planners. We recruited 430 couples by postal letter among 52,255 members of four trade unions. Couples without earlier reproductive experience were enrolled when they discontinued contraception and were followed in six complete menstrual cycles or until a pregnancy was recognized. Data included daily recordings of coitus and menstrual bleeding. Blood, semen, and urine specimens were collected to assess endocrine profile, semen quality, occult embryonal loss, and occupational exposures. Information was obtained about 1661 menstrual cycles (94% of expected). The proportion of couples obtaining a clinical pregnancy during 6 months of follow-up was 65%, and the average probability to obtain a pregnancy across all cycles was 16% (95% CI 14-18%). Among all conceptions the proportion of early embryonal loss was 17% (95% CI 12-22%). This study demonstrates the feasibility of a prospective study of fertility and that the established study base can provide useful information on environmental risk to subfertility.