Professor Allan Pacey, a Sheffield University fertility expert, said: "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 past 20 years.
The fertility calculator table can tell a woman 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 - depending on the size of the family and whether she would try IVF. 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.
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."
Importantly, IVF offers little guarantee that women will be able to conceive later. Factoring in information from women who have had test-tube babies shows that IVF generally only gives a woman an extra year or two.
Perhaps surprisingly, the research found that pregnancy remains an option for women in their early 40s - albeit with odds of 50 per cent. 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."
Adam Balen, chairman of the British Fertility Society, said that family planning should not be just about avoiding pregnancy, but must also tell women about the risks of delaying motherhood.
Earlier this summer, the broadcaster and IVF expert Professor Robert Winston was criticised for saying it was 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 should not wait so long to try for a baby. They also pointed out that those older women who do become pregnant have a higher risk of miscarrying or having a child with Down syndrome.
- Daily Mail
