Women may ovulate more than once a month, suggests a Canadian study that overturns conventional views on the human menstrual cycle.
The findings may explain why the rhythm method of contraception is so unreliable and could lead to improved, targeted fertility treatments in the future.
Ten per cent of the women studied released two eggs in the same month. And all the women examined by researchers at the University of Saskatchewan in Saskatoon, Canada showed at least two waves of maturing eggs in their ovaries in the same month.
Traditionally, it has been thought that ovulation takes place only once in every menstruation cycle. A growth wave of 15 to 20 egg-carrying cells called follicles occurs before ovulation. One follicle will become dominant while the others die off.
“It’s been assumed until now that women have just one wave per monthly cycle, leading to one ovulation, but nobody has actually carried out detailed analysis before,” said Roger Pierson, who led the study. “In fact, all the women in our study had at least two waves and 30 per cent of them had three.”
The team carried out daily, high resolution trans-vaginal ultrasound examinations on 63 women with normal menstrual cycles who were aged between 18 and 40. Over six weeks, the women’s individual follicles were measured.
During the study, 50 of the women ovulated only once, but six ovulated twice and seven not at all. The next stage of the research will be to find out why some waves lead to ovulation while others do not.
“We don’t know why some waves lead to ovulation and others don’t. But we want to utilise our new understanding of the menstrual cycle to be able to carry out better timing of in-vitro fertilization,” Pierson told New Scientist.
Progesterone secretion from the corpus leuteum – a temporary hormone-producing gland that is formed at ovulation – may cause a surge in a reproductive hormone called luteinizing hormone, he says. This might have an inhibitory effect on further egg release during the menstrual cycle, which could explain why some of the women did not ovulate twice despite having two growth waves in their ovarian tissue.
It may be possible in the future to harness some non-ovulating waves into releasing eggs for couples having trouble conceiving, he added.
The unexpected findings may also explain why the conception rate for non-identical twins is as high as 10 per cent.
“It could be that more than one dominant follicle is produced during a wave or that dual conception arises as a result of two waves in a cycle releasing eggs,” ventured Pierson.
Journal reference: Fertility and Sterility(vol 80, p 116)