Scientists have developed a method which they say could predict when a woman will go through the menopause.
The research could help women know how long they will be fertile for
As female fertility starts to decline rapidly around 10 years before this happens, the information could help women decide when to have a family.
Writing in Human Reproduction, the Scottish researchers say it could "revolutionise" family planning.
But they stress more research is needed before women will be able to go to their doctor for an assessment.
The human ovary contains a fixed pool of immature eggs, which form in the fourth month of pregnancy.
They peak at several million in the five-month foetus then start to decline. By birth, this number has already fallen significantly and the decline continues relentlessly.
The number reaches around 25,000, usually at around the age of 37, then the decline accelerates more rapidly.
By the time a woman is about to go through the menopause, this number will have fallen to around 1,000 - too few to generate a mature egg capable of being fertilised.
The experts showed that it was possible to tell how many eggs a woman still had in her ovaries by measuring their size.
On average, a woman will go through the menopause aged around 50, but it can happen as early as 42 and as late as 58.
The team created a model estimating the size of an 'average' woman's ovaries throughout her reproductive life.
Using that model, they say it will be possible to assess whether a woman is likely to have an earlier or later menopause, based on her ovarian size.
Long-term studies will now follow young healthy women - carrying out regular assessments of ovarian volume until they reach the menopause - to carry out a full test of the hypothesis.
The researchers say the information will be useful for women who have cancer treatment when young, which could affect their fertility.
They also say it will help women who are undergoing IVF - as it has been shown that women with smaller ovaries have less chance of success.
But they say it will also be useful for women who are planning to delay having a family, for professional or personal reasons.
Dr Hamish Wallace, consultant paediatric oncologist at the Royal Hospital for Sick Children in Edinburgh, Scotland, led the study.
He said the research "opened the door" to the possibility of screening women to see if they were likely to face an early menopause.
He added: "There is currently no reliable test of ovarian reserve for an individual woman that will predict accurately her remaining reproductive life-span.
The method, he said "may allow us to predict for a woman, aged 25-50, what ovarian reserves she has and at what age she is likely to experience the menopause".
"In essence, it means we now have the potential to be able to tell a woman how fast her biological clock is ticking and how much time she has before it will run down."
The researchers are also studying young women successfully treated for cancer to hopefully give them better fertility advice - allowing them to realistically plan a family.
HOW IT WOULD WORK
WOMAN A - Aged 40 but has ovary size, and therefore reproductive age, of an average 36-year-old
= menopause at around 54/55
WOMAN B - Aged 40, but ovary size and therefore reproductive age of a 44-year-old
= menopause at around 48 or 49
Professor Bill Ledger, a spokesman for the Royal College of Obstetricians and Gynaecologists, told BBC News Online: "What we're trying to do is tell women how much fertile time they have left."
He said that, while women in their 40s could become pregnant, it was rare because their egg quality declines.
He added: "Most, although not all, women have in their minds that they will have children at some point.
"They want to know what their individual chance of becoming pregnant is, so they can plan."
A spokeswoman for the Teenage Cancer Trust said: "Fertility is a major concern for survivors of teenage cancer as their fertility is often affected by their treatment.
"It will be of great benefit to these survivors to assess the extent of the damage to their fertility caused by their treatment, and will allow them the opportunity to know whether they can realistically start planning a family."