As fertility-related traits fluctuate across time (see the electronic
supplementary material, figures S1 and S2), 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 restricted
our 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.