The odds of getting pregnant after a certain time trying are surprisingly hard to come by. There are statistics here and there, but none provide a good overview of the probabilities. Mathematician Richie Cotton crunched some numbers using monthly fecundity rate — the monthly chance of getting pregnant — to estimate about how long it would take for he and his girlfriend to conceive.
[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.
Obviously, there are other factors to consider like male fertility and how often a couple has sex, but there you go.
Shouldn’t the probability of conception after 60 months for a 35 year old be the same as the probability of conception after 0 months for a 40 year old?
No, but good question. The 35 year old has been trying for 60 months straight. The 40 year old is just getting started. The probability over time is additive. Even though the 35 year old at 60 months is at the same AGE as a 40 year old, the circumstances described by this study are very different for the two.