A woman’s fertility starts to decline steadily from age 20 to 30, with a rapid decline after she hits 35. Between age 20 to 30, the probability of conceiving declines from 30 to 25 per cent every month.
By age 40, the probability is less than five per cent, and negligible after age 45.
This is because a woman is born with all the eggs she will ever have, starting with six to seven million eggs as a foetus. This is in contrast with men whose ability to produce sperm is limitless, with average sperm production going up to 100 million per day.