The conventional belief that women ovulate once a month is wrong, say Canadian researchers who have found that women can potentially ovulate two or even three times a month.
Figure 1.
Micrograph of a mature ovarian follicle. Ultrasound scans on 63 women found that they all had at least two waves of follicular development
Credit: ASTRID AND HANS-FRIEDLER MICHLER/SPL
The research, published in the journal Fertility and Sterility (2003;80: 116-22), could explain why the “rhythm” method of contraception is so unreliable and why women who take hormonal contraceptives sometimes become pregnant.
Researchers from the University of Saskatchewan did daily ultrasound scans on 63 women who apparently had normal menstrual cycles. Some were nulliparous; others had had up to three children. They found that all of the women produced at least two waves of follicular development. The existing theory held that at the beginning of each menstrual cycle, 15 to 20 follicles begin to grow in the ovaries and that one of them develops into a mature egg at roughly the middle of the cycle.
Current scanning techniques can detect follicles but cannot reveal the much smaller egg itself, so it is unknown whether any of the women actually ovulated twice.
Dr Roger Pierson, director of the reproductive biology research unit at the University of Saskatchewan, who led the study, said 40% of the subjects had the clear biological potential to produce more than one egg in a single month. Moreover, they could be fertile at any time of the month.
“This really isn't the result we expected, and if it's confirmed we'll have to rewrite the textbooks,” he said. “It explains why natural family planning often doesn't work, why hormonal contraception sometimes fails, and why we see fraternal twins with different conception dates.”
The findings open the prospect of new approaches to contraception and assisted reproduction, Dr Pierson says. Women who take drugs to harvest eggs for in vitro fertilisation could potentially yield many more oocytes in a given month. The study also calls into question the rationale for including a week of placebo pills in a monthly cycle of oral contraceptives.
Professor Robert Winston, head of the department of reproductive medicine at the Hammersmith Hospital in London, said the findings were “logical and not altogether surprising.”