Women who want to have a big family should start trying for a baby by 23, experts have warned.
Even those who only want one child shouldn’t wait past the age of 32, if they really want to become a mother.
The advice comes from scientists who crunched together fertility data on more than 58,000 women to create an at-a-glance calculator.
It tells a woman the best age to start a family – and makes it clear that IVF is not a guarantee for those who wait.
British experts said that the table is so useful that it should be shown to sixth formers and university students to underline the risks of delaying motherhood.
Professor Allan Pacey, a Sheffield University fertility expert, said: ‘We haven’ t got a time machine we can put people in….that’s just a blunt reality.
‘Everyone thinks you can wait – this shows you can’t.’
The advice comes as growing numbers of British women delay motherhood until they establish their career, become financially stable or find ‘Mr Right’.
Around half of all babies are now born to women aged 30 and older, and the number of children born to women aged 40-plus has trebled in the last 20 years
To use the fertility calculator, a woman decides how many children she wants and whether she is going to try to conceive naturally or with IVF.
The table then tells her at what age she would need start trying for a family to have a 90 per cent, 75 per cent or 50 per cent chance of success.
For instance, a woman who starts trying at 23, will have a 90 per cent chance of having three children, if that’s what she wants.
If she waits until she is 31, her odds of becoming a mother-of-three fall to 75 per cent.
And if she holds on until 35, they drop to just 50 per cent, this week’s New Scientist reports.
WHEN TO GET STARTED: THE FERTILITY CALCULATOR
CHANCE OF SUCCESS
Natural conception
90%
75%
50%
With IVF
90%
75%
50%
1-child family
Age 32
37
41
35
39
42
2-child family
27
34
38
31
35
39
3-child family
23
31
35
28
33
36
Source: New Scientist
Professor Pacey said that while 23 may seem young, the figure is sensible.
He said: ‘You’ve got to factor in that people don’t necessarily have children in quick succession.
‘What it is saying is that if you’re relaxed about having three children, you can wait until you are 35, but you’ve got to start early to be certain.’
A woman who wants two children should start by 27, to have the best chance of success, while 32 is advised for those who would be happy with just one baby.
Importantly, IVF offers little guarantee.
Factoring in information women who have had test-tube babies shows that IVF generally only gives a woman an extra year or two.
There are other surprises. For example, pregnancy remains an option for women in their early 40s – albeit with odds of 50 per cent.
‘Everyone thinks you can wait – this calculator shows you can’t,’ said leading fertility expert Professor Allan Pacey
The figures are averages and so won’t apply exactly to every woman but doctors say they could still be very helpful.
David Keefe, of New York University, said: ‘It makes explicit certain statistics that don’t sink in for many people.’
One of the calculator’s creators, Dik Habbema, of Erasmus University in Rotterdam, said: ‘We have tried to fill a missing link in the decision-making process.
‘My son is 35 and many of his friends have a problem deciding when to have children because there are so many other things they want to do.’
Professor Pacey said: ‘The table ought to be photocopied and put on the clinic wall.
‘We should be aiming this at sixth formers and university students, so that they’re aware of how to plan their life.’
Adam Balen, chairman of the British Fertility Society, said that family planning should not be just about avoiding pregnancy, it must also tell women about the risks of delaying motherhood.
Society must also do its bit.
Professor Balen told New Scientist: ‘Couples need support so they can start their families early.
‘Women who have children in their 20s are more likely to achieve their desired family size but can also expect lower lifetime earnings than women who start later.
‘We need to ensure women aren’t disadvantaged at work and sort the lack of childcare facilities so we can enable young people to establish their careers and families at the same time.’
Earlier this summer, the broadcaster and IVF broadcaster Robert Winston was criticised for saying good to delay motherhood. Lord Winston said older mothers, who have had time to gain skills and education, as well as build strong relationships, can provide children with a more stable upbringing.
But others warned that age-related declines in fertility mean older women could remain childless.
And pointed out that those who do become pregnant have a higher risk of miscarrying or having a baby with Down’s syndrome.
