Women are being offered a new test kit that allows them to predict the rate at which their fertility is declining.
More women are trying for families later in life
The test works by measuring the number of eggs in a woman's ovaries and indicating what that level might be like in two years time.
It is hoped the test, designed by fertility experts at Sheffield University, will help women decide how long they can delay trying for a baby.
The Plan Ahead test is only available privately for £179 and not on the NHS.
Sheffield fertility expert Professor Bill Ledger, who developed the test kit over five years, said he was confident it was an accurate predictor of fertility and that he hoped it would allow women to plan for the future.
"If having family is the most important thing, you're best trying to have babies when you're in your twenties - there's no question that the longer you delay, the bigger your chance of heartbreak," he said.
"But I think the reality of modern life is that women in the Western world, for all sorts of good reasons, are putting off starting a family until they are in their thirties."
He said: "There is more research going on but this is good enough to do what it says it does - give people a two-year warning that their ovarian reserve is declining."
Critics have voiced doubts about the test, pointing out that lack of fertility can have complex causes.
The test measures the levels of three fertility hormones in the blood and plots the results against average fertility for the woman's age.
On the third day of her period, a woman gives a 3ml sample of blood, which is sent off for laboratory testing to measure the levels of the hormones.
The first two are Inhibin B and AMH, which decline as the menopause approaches.
The third is a pituitary hormone known as FSH - this tends to increase when the menopause nears.
A combination of the three will indicate the woman's reserve fertility, scientists say.
This is then plotted onto a graph showing the woman's position compared with the average fertility for women of the same age.
The predictive nature of this test means that the woman's ovarian reserve can be predicted for the next two years, says manufacturer Lifestyle Choices which is linked to the University of Sheffield.
Fertility is known to sharply decline from the age of 35, on average, but some women's ability to have children falls even earlier.
One in 100 women have their menopause by the age of 40, but fertility begins to decline a number of years before its onset.
Clare Brown, chief executive of Infertility Network UK, said the test was a good way of allowing women to make a more informed decision about when to try for a family.
But she added: "It is important to realise that whilst Plan Ahead is a valuable test, there are other factors that may cause difficulties in conceiving so it is vital that men and women make sure they are fully informed when making decisions on when to have a family."
Dr Susan Bewley, pregnancy specialist at Guy's and St Thomas' Hospital, recently warned in a British Medical Journal article that women risked heartbreak if they delayed attempting to conceive.
She said: "The only reliable test of fertility is actually trying to get pregnant. Test kits are likely to lead to people putting off that process and can give false reassurance.
"Couples would be better off spending their money on a candlelit dinner."
Dr Allan Pacey, secretary of the British Fertility Society, said: "This will be a useful test for those women who want to be able to better plan when they want to have their children.
"However, they should remember that female fertility declines with age and that there are dangers of leaving things too late in life.
"IVF doesn't work for everyone and there are dangers in thinking it can."