Background: Little is known about time trends in fecundity because few population-based data are available. In a survey among female twins born from 1953 to 1976, their time to pregnancy did not differ from singletons and can be considered to represent the fecundity of the general population.
Methods: Information was collected by interview about waiting time to first pregnancy (TTP) and any periods of subfecundity among both male and female twins born between 1931 and 1952. Trends were analysed by considering the year of birth of the index person (birth cohort effect) or year at which the first attempt started (period effect).
Results: Eighty-five percent of male and 81.3% of female eligible twins participated. A total of 1598 male twins and 1653 female twins reported a TTP value and 1671 men and 1715 women had a value for their first attempt to conceive including unsuccessful attempts. No overall trends in either male or female TTP were observed with increasing year of birth or of starting time, after adjustment for confounders, but for attempts fecundity increased among female twins by year of birth or of starting the attempt. A decreasing risk of severe infertility with increasing year of birth or year for start of the attempt was observed among male twins, but this observation was based on only 81 men and was not seen among female twins.
Conclusion: No decreasing trend in fecundity was observed among Danish twins born between 1931 and 1952 who had completed their reproduction. Female twins had a slight increase in fecundity, and men a decrease in severe infertility. In addition, TTP was apparently well reported and recalled for up to 50 years among both male and female twins.