A discovery about what happens inside a woman's ovary during her monthly cycle could one day improve IVF treatments, say scientists.
Women may produce more than one egg a month
Certain changes in the ovary were thought to happen only just before ovulation - the release of an egg for fertilisation.
In the days leading up to this, cells called follicles carry mature eggs to the surface of the ovary, and then rupture, releasing the egg into the fallopian tube.
However, researchers have now found these follicles develop at other times of the month, when eggs are not released.
These "follicular waves" could be harnessed during fertility treatment and provide more opportunities for treatment than the existing single "peak", they say.
It could, for example, mean the amount of fertility drugs women have to take during IVF could be cut back.
Researchers from the University of Saskatchewan carried out daily ultrasound scans of 50 women for a month, allowing them to examine the follicles closely.
All had normal menstrual cycles,
Thirty-four women had two waves of follicle development, while the other 16 had three waves.
In all the women, an egg was released only during the final wave.
Writing in the journal Fertility and Sterility, the researchers said: "We anticipate that the knowledge of follicular waves during the menstrual cycle will have profound implications for infertility diagnoses and treatment in women.
"More than one wave of follicular development during a woman's cycle may provide women undergoing assisted reproduction with better opportunities for initiating ovarian stimulation protocols.
"This option would provide women with a more time-efficient and less expensive treatment regimen.
"Consideration of a wave model for ovarian follicular growth may also be useful for the development of more efficacious and user-friendly hormonal contraceptive regimens."
More research is planned to follow women for several months in a bid to find out the cause of extra follicle production.
Professor William Ledger, an expert in ovulation at the University of Sheffield, told BBC News Online: "The researchers have identified waves of activity in the ovary. Follicles are growing in these ways through the whole cycle."
But he said: "There is still only one ovulation per cycle.
Professor Ledger, a member of the Royal College of Obstetricians and Gynaecologists, said any research in this area could help understanding of why some women experience fertility problems.
"Each follicle that grows and doesn't release an egg, they just grow and die.
"If women have three waves of follicle activity, and others have two, those who have three may get through their lifetime supply more quickly."
He added: "This work could also help understanding of why some women go through an early menopause."