The decrease in fertility with age is a biological reality, but I also suspect it's influenced by attrition selection and stress, and looks worse because of it. Fecundability seems to be roughly linear with age, and gravid women have significantly higher fecunability ratio from 40-45 than nulligravid women do.
> In this preconception cohort study of North American pregnancy planners, increasing female age was associated with an approximately linear decline in fecundability.
> In this preconception cohort study of North American pregnancy planners, increasing female age was associated with an approximately linear decline in fecundability.
https://www.ajog.org/article/S0002-9378(17)31107-9/pdf