A leading expert on falling birth rates has said that if a couple want to have two or more children, they should start trying in their 20s, rather than wait until their 30s, to maximise their chances.

Michael Hess who tweets under the handle ‘More Births’ cited a study that showed when the woman is in her 20s the odds of being able to have two or more children is greater than 90pc.

However, by the time she is 35, 1/3 of couples will be unable to have two children naturally.

To have very strong odds of success (>90pc), the authors of the study he cites write, “couples should start no later than age 32 years for a one-child family, at 27 years for a two-child family, and at 23 years for three children.”

The study also shows that IVF doesn’t change the fertility curve all that much.

“The fertility gain from IVF is far less than the fertility gain from simply starting in the 20s”.

For those who want just one child, the curve is more forgiving.

Women who start at 40 still have a 60pc chance of having at least one child naturally – and that rises to 70pc with IVF.