Mathematician Richie Cotton has been crunching the numbers since he and his girlfriend started trying to get pregnant:
[A]lmost half of the (healthy) 25 year olds get pregnant in the first month, and after two years (the point when doctors start considering you to have fertility problems) more than 90% of 35 year olds should conceive. By contrast, just over 20% of 45 year old women will. In fact, even this statistic is over-optimistic: at this age, fertility is rapidly decreasing, and a 1% MFR at age 45 will mean a much lower MFR at age 47 and the negative binomial model breaks down.
(Hat tip: Nathan Yau)
The Dish 