Some things in life are immutable. There is a rhythm to the seasons, night follows day, tides ebb and flood and a woman's body goes through a menstrual cycle every 28 days or so until the menopause sets in. That is the way it has always been and we assume it will forever stay.

But does it have to be so? Or are we needlessly imprisoned by our and our doctors' assumptions about what is natural? Modern woman has nearly three times as many periods as her forebears endured, and evidence is accumulating that the toll of all this menstruation may not only be nuisance and distressing mood swings, but also an increased propensity to some diseases.

The first myth is that modern menstruation patterns are natural. In an article in the latest Lancet medical journal, Charlotte Ellertson and Sarah Thomas, based in the Mexico office of the population council, write that times have changed: "Women in pre-historic times, as estimated by research among contemporary hunter-gatherer populations, probably had far fewer periods (about 160 ovulations over their lifetime) than modern women."

Women living in industrialised countries today begin menstruating at an earlier age and go through the menopause later. They limit their families to two or three children and scarcely breastfeed, Ellertson and Thomas say, "three months per birth, with half of American infants never breastfed at all". So their bodies get very little break from menstruation, with its hormonal surges and the physical stress on the ovary each time a ripe egg bursts through the wall. Modern woman can expect to go through about 450 periods in her life.

So it can be argued that women are living an unnatural life - a sophisticated life of our own choosing, thanks to modern contraception, but one which leads to an unnatural number of periods. Some women are relatively undisturbed by their monthly bleed. Some welcome it as a sign of their fertility or as proof that they are not pregnant. But for some, periods are painful and debilitating - the "curse" of generations of women.

The Lancet article argues that "there is plenty of modern evidence that amenhorrhoea (absence of periods) is often healthier than the alternative". A woman who is not ovulating escapes the sharp changes in hormone levels that can lead to mood swings. Arthritis, epilepsy and other conditions that worsen with the cycle would remain stable. Menstruation can cause endometriosis and anaemia.

But most alarming is probably the link with ovarian cancer, explored in the current New Yorker magazine which, coincidentally with the Lancet, has taken a radical look at menstruation and the pill. Ovulation, it points out, encourages cell division as the breach in the ovary wall caused by the release of the egg is healed. Studies have shown that a woman's lifetime risk of ovarian cancer drops by 10% every time she has a child.

"Why? Possibly because, between nine months of pregnancy and the suppression of ovulation associated with breastfeeding, she stops ovulating for 12 months and saves her ovarian walls from 12 bouts of cell division," says the article.

The same argument applies to the changes in the lining of the womb, potentially increasing the chances of endometrial cancer, which, together with ovarian cancer, is a characteristically modern disease, the consequence, in part, "of a century in which women have come to menstruate 400 times a lifetime," it continues.

Dr Sheldon Segal, a distinguished endocrinologist, well-known contraceptive researcher, and co-author of Is Menstruation Obsolete?, agrees with this scenario. "The evidence is very clear", he says, for the impact of menstruation on ovarian and endometrial cancer, although the jury is still out on the possibility that it might be implicated in breast cancer. "Among women who take oral contraceptives, they reduce their risk of ovarian cancer by one seventh," he said.

But they still have periods, don't they? Well no, they don't. Not a menstrual period in the proper sense of the words. What they have is a superficial bleed, which is not the same thing. There is no ovulation. This is the irony that the New Yorker hooks into. That for women on the pill, that bleed is not necessary.

It was devised by a pioneer of the pill, John Rock, who thought that if women took 21 pills and then seven placebos, enabling them to have this non-functional bleed which seemed like a period, then the Catholic church might approve the pill as a "natural" form of contraception which did not interfere with the normal rhythms of life. All it did, he argued, was extend the natural, infertile, "safe" period.

Of course, the argument was kicked into touch by John Rock's church. But the pill continued to be made and marketed in the pseudo-natural way he had devised. It may be that many women feel better about taking the pill knowing that their body has a week's break, but in fact, argue today's scientists, it makes very little difference. "We have never had any reason why there is a need for a rest," said Dr Segal. "It was more of a means of selling the idea to women rather than there being a medicinal basis for three weeks on and one week off."

For all that he, and the Lancet writers, are challenging the orthodoxy, Dr Segal says he is not urging all women to take the pill continuously and give up periods for good. "I do see it as a possibility that women will want to have fewer periods as a lifestyle choice, but I'm not really urging women to do this if they have no problem with their menstruation. But when it stands in the way of their normal functioning, women should be given the opportunity to decide for themselves."

Ellertson agrees. Speaking from the labour ward of a hospital in Mexico where she was about to give birth she said: "I don't think women would want to use the pill for decades on end - probably just for a few months and then have a break. But there is no evidence that the placebos do anything good."

Some women have been aware that they can take two packets of pills in a row to avoid a period interrupting a special event, such as a holiday. But they have a sense that they are doing something irregular and possibly damaging. It is also, says Dr Segal, only a certain group of women who generally are that well informed. "In the United States, that wisdom is mainly available to middle-class women with their own private physician," he said. And not all doctors or family planning clinics would give that advice.

Ellertson and Thomas, who say women who think they are maintaining a natural cycle on the pill "are being duped", think women should be given the information so they can decide what sort of menstrual pattern suits them. "Women could use pills to adjust their cycles as convenient," they suggest. In the future, they say, "women most harmed by their cycles might choose to suppress their periods for years at a time. Others might simply reduce their annual number of bleeds."

For all the regular scare stories about the pill in the UK, it is one of the best-studied drugs in medicine, she points out. Women whose periods distress or damage them should be told there is that option.

"Health professionals and women ought to view menstruation as they would any other naturally occurring but frequently undesirable condition," she writes in the Lancet. "This means providing those women who want it with safe and effective means to eliminate their menstrual cycles, contributing to happier, less encumbered lives and helping women individually and society as a whole."