As fertility-related traits fluctuate across time , we stratified birth year into 5-year cohorts (e.g. 1935–1939, 1940–1944, . . ., 1965–1967).
We excluded birth cohorts prior to 1935 owing to insufficient sample size (excluding 846 cases). We also included only those men and women who were 45 years of age and older (as virtually all of these have completed reproduction), which further restrictedour sample to the birth year, 1967.
Many women show perimenopausal symptoms at this age accompanied by higher levels of sterility [20]. In this sample only 0.1% of women reproduced above the age of 45. Although men are physically able to reproduce to a very late age,we observed that only 1.7%of men in our sample
had their last child after the age of 45.