Researchers have created a fertility calculator which predicts the likelihood of having a family based on a woman’s age

Women who want three children should start having kids by the age of 23, according to scientists.

If they are content just to have one child, they shoud not leave it later than 32 for a 90 per cent chance of success.

Dutch researchers have created a fertility calculator which predicts the likelihood of building a dream family based on a woman’s age.

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Those who want two children should start aged 27 to have a 90 per cent success rate, it predicts.

If they leave it until they are 38, the likelihood falls to less than 50 per cent.

The average age for starting a family in England has steadily risen over the past four decades as increasing numbers of women delay motherhood while they follow their careers.

It is now 30, up from 27 in 2004. In 1970, it was just 24.

Scientists at the University of Rotterdam looked at the data on the natural fertility of 58,000 women spanning 300 years up to the 1970s.

They calculated that women who wanted three children would have a 90 per cent chance of success if they started at the age 23.

If they only wanted one, they would still have a 90 per cent success rate if waited until 32 but after the age of 41 the chances were less than 50 per cent.

Dr Dik Habbema, from Erasmus University in Rotterdam, and who devised the calculator said: ‘It’s not easy to make recommendations.

‘I hope the model will play a part in making decisions easier.’

Dr David Keefe, from New York University’s Langone Medical Centre, said women in their thirties who still wanted to have children need not panic.

‘Some might think, "oh my gosh, I have to get started", but for many women that’ s not true,’ he told New Scientist magazine.

‘For women at the age margins, this could help nudge them one way or another, but we don’t want to force people to change their lives.’

But other experts said women had a very short amount of time to ‘act’ if they wanted to establish a career and also start a family.

Jody Day, the founder of Gateway Women, a support organisation for those who are childless through circumstances said: ‘It’s a very tall order.

‘Women have moved into a pattern of work created over the years around men and it’s not really suited to them.

‘Women have a very short amount of time in which to act if they want to go to university, get a job and establish themselves in a profession and have financial stability before starting a family.’

Professor Allan Pacey, a fertility expert at the University of Sheffield said the calculator should be used to enforce the message to girls at sixth form and university to help them plan their lives.

‘Everyone thinks you can wait – this shows you can’t. The table ought to be photocopied and put up on the clinic wall.

‘We should also be aiming this at sixth formers and university students, so that they’re aware of how to plan their life.’

Earlier this summer scientists warned that celebrities who had given birth in their 40s through IVF or donor eggs were giving ‘false hope’ to women.

Dr Marta Devesa, of the Hospital Universitari Quiron-Dexeus in Barcelona, told a fertility conference that by the ages of 38 and 39 the success rate was less than a quarter.

‘The prognosis is really futile from 44 and onward,’ she said. Yet the growth of delayed motherhood means a fifth of British IVF patients are in their 40s.

‘There is not a lot of knowledge about fertility’s decline with age in the general population,’ she added.

‘And it is true that pregnancies in older women and celebrities don’t help us because many people don’t know that most of the time they have gone for egg donation.’