Menstruation may be a defence against bacteria,; 'Menstrual blood seems to have an antiseptic function'
Why do women bleed? Menstruation has been hailed as everything from a wise wound to a curse, but why it occurs at all is still a mystery. Women waste 10 to 80 millilitres of blood every month, plus the same amount of tissue and associated nutrients. The conventional answer, as one textbook puts it, is menstruation is the "penalty women pay for a greater state of readiness for the implantation of embryos". But why not just reabsorb the lining of the womb? Why is it necessary to flush it all out?

These might sound like childish questions to be answered with a firmly adult "because that's the way it is". But they inspired American researcher Margie Profet to come up with a controversial theory about menstruation. She believes it is part of an essential defence mechanism against the bacteria that hitch a lift on passing sperm on their way to the womb.

The theory has implications for the way we treat infections in the womb and on the effects of the Pill and IUDs, as well as questioning the wisdom of having sex during non-menstruating times such as pregnancy and breast-feeding.

Unless you put it all down to God, the way to answer "why" questions about human design is in terms of evolution. Genes for features that made Stone Age ancestors more likely to survive were the ones that kept on being reproduced. So, from an evolutionary point of view, menstruation is likely to have a purpose, otherwise it would have been weeded out.

"Menstruation is not an accident or a design flaw," claims Ms Profet, an independent researcher at the University of Washington in Seattle. "It's controlled by spiral-shaped arteries and the blood that comes out is slightly different from ordinary blood - it has less iron and doesn't clot. The reason for these changes is that menstruation does a specific job: it is an important line of defence against the viruses and bacteria which can be carried up into the uterus by hitching a ride on passing sperm."

Most of these pathogens are already in the vagina or cervix, where they do little harm; a normal number is 100 million to 10 billion organisms per millilitre of vaginal fluid. But transported to the womb they can cause spontaneous abortions or infertility. Studies with animals have shown that after sex the number of bacteria in the womb increases.

The vagina and the uterus have to manage a tricky balancing act. They have to guard against bacteria and other pathogens without destroying the sperm as well. They do this with a cascade of defences, such as producing secretions that are acidic, of which menstruation is the most visible and dramatic in the days in the cycle when there is no chance of pregnancy

"The unique features of menstrual blood all point to its antiseptic function," says Ms Profet, who backs her theory with three biological examples: there is less iron in menstrual blood, which makes it more resistant to certain bacteria; menstruation is always accompanied by a rise in the amount of a chemical called lactoferrin, which absorbs iron; and the blood does not clot, enhancing its ability to wash out infected womb tissue.

Ms Profet admits she is an academic maverick with several controversial studies to her name involving allergies and morning sickness. But her ideas are not so extreme - a grant of $250,000 supports her work. With degrees from Harvard (political philosophy) and Berkeley (physics), she was taken up by the American toxicologist Bruce Ames, professor of biochemistry and molecular biology at the University of California, Berkeley, and given space at his laboratory.

If her poisonous sperm theory works and human wombs are under constant threat of infection, then those of other mammals must be similarly at risk. From other research Ms Profet has come up with numerous examples of wild animals that also have uterine bleeding. But she admits that the menstruation patterns of other mammals are under-researched and much work needs to be done to prove her thesis.

For instance, there are no human studies to show if the number of pathogens in the uterus are greater before menstruation than after it. What is generally agreed, however, is that humans menstruate more copiously than almost any other species and this, says Profet, is essentially because we have more recreational sex.

"One of the unique features of the human sexual set-up is what is known as concealed oestrus: that is, there is no way of telling when the female is fertile. That means even with monogamous couples there is likely to be sex all month round, so bacteria may have over three weeks to establish themselves," she says. Then there is promiscuity, likely to make the pathogens more virulent, because the faster breeding ones are more successful.

So what does all this mean in practical terms? Our relaxed attitude to sex means that intercourse all through pregnancy and while the woman is nursing is quite common. But these are precisely the times when there is no menstruation to wash out any sperm that do get through.

"The mucus plug in the cervix is pretty dense during pregnancy," says Ms Profet, "but it thins out during the last two months." There is some cervical protection during lactation but it is not reliable. Post-menopausal women who enjoy a regular sex life do not have the advantages of this monthly bleed.

The implications for the Pill are complicated. It reduces the amount of bleeding, but it may not increase the risk of infection because the cervical mucus becomes more hostile to sperm. A further factor is the complicated relationship between some infections and sex hormones. Oestrogen, for instance, stimulates chlamydia while progesterone slows down gonorrhoea.

This means that using contraceptive chemicals to suppress regular bleeding should be approached with care. Any woman with a sexual history probably benefits from the additional protection menstruation offers.

The sperm-pathogen theory throws a new light on the problems many women have with IUDs. Ms Profet says: "Doctors have long been puzzled why these cause so much bleeding. But since they inflame the womb, the body's reaction is the same as to an infection. If menstruation is a natural defence, doctors should think carefully before rushing in to stop heavy bleeding."

'Evolution and Healing , the new science of Darwinian medicine' by Randolph M Nesse and George C Williams (published by Weidenfeld & Nicolson, £20 on 25 May) includes a discussion of Ms Profet's work.