A CONTROVERSIAL study has found that women can ovulate more than once per cycle, which implies the textbook explanation of female reproduction will have to be rewritten. The study also suggests a possible explanation for why some women reach the menopause early, and why non-identical twins are so common.
Roger Pierson and his team at the University of Saskatchewan in Canada carried out daily ultrasound scans on 63 women for six weeks. The women all had normal menstrual cycles and were aged between 18 and 40. The team analysed the ultrasound images to measure the diameter of every follicle, or egg-carrying sac, within the women鈥檚 ovaries.
Before ovulation, a cluster of around 15 to 20 follicles swells up and rises to the surface of the ovary. One of these follicles outgrows the others, bursts and releases its egg into the fallopian tube (see Picture). The other eggs then die off. Until now, women were assumed to have just one of these follicle growth 鈥渨aves鈥� per cycle, and only one ovulation. 鈥淏ut nobody has actually verified this claim,鈥� says Pierson.
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All the women in the study had at least two follicular waves and 32 per cent had three (see Graph). During the six weeks, 50 women ovulated once, as expected, and seven did not ovulate at all. But the most surprising finding was that six ovulated twice in the space of a month, on two separate days. This double ovulation could explain why non-identical twins (when two eggs are fertilised in one pregnancy) account for 10 per cent of conceptions (Fertility and Sterility, vol 80, p 116).
William Ledger, an expert on reproduction at the University of Sheffield in the UK, is impressed by the findings. 鈥淭he thing that surprised me is that no one thought to do it before,鈥� he says. Ledger thinks the findings may explain why some women reach the menopause much earlier than others. 鈥淚t is possible that women who have three follicular waves per month run out of eggs faster than those who have two.鈥� If a genetic link is established, he adds, then a test could be developed that tells women how much longer they will be fertile for.
Pierson now plans to explore why some waves lead to ovulation while others do not. He thinks the progesterone hormone secreted at ovulation causes a surge in luteinising hormone, which might inhibit the release of another egg during the menstrual cycle.
But Richard Kennedy of the British Fertility Society is sceptical. 鈥淭his flies in the face of existing knowledge,鈥� he says. 鈥淔ollicular development has been intensively studied for decades. It would need to be verified independently.鈥�