Researchers say, women are less likely to conceive after the age of 30 as their fertility levels decline by 12pct.
The team from St Andrews University and Edinburgh's Royal Hospital for Sick Children has found that the number of eggs in the ovarian reserve peaks at about 20 weeks after conception - when the female embryo is still in her own mother's womb - and plunges until menopause at the age of about 50.
They said women are born with an estimated 800,000 eggs.
But around the age of 37, when there are 25,000 eggs left, the decline speeds up even more and the ovaries shrink until most of the eggs are depleted.
Menopause occurs when they have less than 1,000 eggs left.
The researchers said that monitoring the speed of the biological clock will enable doctors to predict how long a woman will remain fertile and whether she will have an early menopause.
Early warning would help the women start a family earlier or freeze some eggs to increase the chances of conceiving in her 30s.
"A better understanding of the dynamics of ovarian reserve will help us predict which young people treated for cancer are most at risk of an early menopause," the Daily Express quoted Hamish Wallace, a consultant oncologist at Edinburgh's Royal Hospital for Sick Children, as saying.
"These patients may benefit from having their eggs frozen before cancer treatment starts," Wallace added.