Dear Economist,

I’m an ambitious woman in my mid-twenties, just starting what I hope will be a stellar career in business. But I also very much want to have at least one child. How long should I leave it?

Yours sincerely,

Ms E. Jones, West London

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Dear Ms Jones,

Mothers seem to do worse in the labour market than women without children, but that might not be simple cause and effect. For instance, it might indicate that women who expected successful careers delayed having children, but the delay was not the cause of the success.

It all seems imponderable, but it isn’t. Amalia Miller, an economist at the University of Virginia, studied the timing of maternity and its effect on earnings. That effect is large: delay maternity by just one year and you can expect your career earnings to rise by 10 per cent, partly because you will work longer hours and partly because you will enjoy a better wage rate. For professionals like you, the wage effect is even higher.

These numbers strip out the effects of choice, because they are all based on accidents. Professor Miller made three types of comparison. She compared women who became mothers at 27 with those who became mothers at 28 despite both groups using contraception and therefore not choosing the timing. She also compared women who successfully got pregnant at 27 with women who tried at 27 but did not succeed for a year; or women who miscarried at 27 and then got pregnant again a year later. The women wanted the same date of pregnancy but bad luck intervened - and their careers benefited.

Professor Miller’s results suggest that you should leave your pregnancy as late as you dare. Her methods remind you, though, that you may not get to decide.

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