They may also need more alcohol than usual to experience its pleasurable effects in the middle of their menstrual cycle

They may also need more alcohol than usual to experience its pleasurable effects in the middle of their menstrual cycle.In a review of research on women and alcohol, the Netherlands Research Group on Addictive Behaviours found that levels of alcohol in the blood appear highest in the pre-menstrual phase, usually defined as the four days before the onset of menstruation, and that blood alcohol levels are particularly high immediately before the onset of the menstrual flow.At this time, it appears, a woman's body is least able to eliminate alcohol. Many women who like a drink notice that their ability to handle alcohol varies through the month, but the evidence for this has always been largely anecdotal. Now researchers from the Netherlands have brought together a body of evidence that suggests these social observations are based in biological fact. Women may indeed be less able to handle the effects of alcohol in the four days leading up to menstruation than at other times. 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..

But since they inflame the womb, the body's reaction is the same as to an infection. Ms Profet says: "Doctors have long been puzzled why these cause so much bleeding. 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. It reduces the amount of bleeding, but it may not increase the risk of infection because the cervical mucus becomes more hostile to sperm. 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. 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. 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. 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. 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.