But despite all the anecdotal evidence, it was not until 1971 that it was proven to be a biological phenomenon, when Professor Martha McClintock conducted the first scientific research into it. McClintock studied 135 women living together at an American college. Over one academic year the students kept ''menstrual diaries''. She found that whereas at the beginning of the year they all started their periods at different times, by the end of the year they had all synchronised to within a few days of each other.
Since then, McClintock's studies have been replicated by various others. Today at the forefront of this research are father and son team Leonard Weller and Arran Weller from the Bar-Ilan University in Israel. In 1992 they looked at the cycles of 20 lesbian couples and found that their periods synchronised ''very frequently'' with half of them menstruating within two days of each other. In 1993 they studied mothers and daughters, and room-mates. The mothers and daughters especially ''displayed a significant degree of synchrony''. However two years later when they studied women working together - 72 women soldiers and 36 civilian office workers - no ''signficant synchrony'' was found. But they did find that friendship, mutual activity and length of time spent working together played a part in it.
Cynthia Graham, an adjunct assistant professor of psychology and psychiatry at Indiana University, researched synchrony ten years after McClintock's findings were published. She also believes that ''emotional attachment'' plays a large part. "Over the four months that I did my study, I found that menstrual synchrony was documented in women who were close friends," she says. "There is some suggestion that it is not just time spent together but the emotional attachment, the relationship between the women." So just living or working together isn't enough, but the closer you are with a woman the more likely you are to synchronise: women who bond together bleed together. Proven it may be, but despite all this research no one yet knows how it happens.
The most likely reason is that synchrony is mediated by some kind of olfactory communication via chemical messages. In an attempt to find answers, Professor McClintock studied rats (who do not menstruate; she studied their oestrus cycles). She fed air from one cage to another and discovered that rats could send airborne messages to each other and that in so doing, they could alter each other's cycles. The rats that had already ovulated could send ''slow down'' messages to the rats that hadn't and those rats that had ovulated could send ''hurry up'' messages to speed up ovulation in others. In a human experiment, volunteers had the sweat from the armpits of a woman with regular periods daubed on their upper lips. After four months the volunteers had started to synchronise with the woman whose underarm secretions they had been sniffing.
Whether one woman does actually ''lead'' in real life, is as yet unknown. "We don't know whether women converge together or one woman leads. There's been a lot of speculation about this, but no good studies,"explains Graham. Although it takes about four cycles to start synchronising, in any community about 30 per cent of the women will have ''independent'' cycles. Not being ''close'' to the women they live or work with is just one reason why they might not be influenced. Men, it seems, can also throw a biological spanner into the works. Sherri Matteo, a psychologist at Stanford University, California, found that women living with their male partners do not become synchronised, most probably due to male odour upsetting whatever it is that causes synchrony in the first place.
Why women should start bleeding together is a mystical question. But there is a theory. In his book Blood Relations: Menstruation and the Origins of Culture (Yale University Press, pounds 10.95) Chris Knight, lecturer in anthropology at the University of East London, discusses his theory on the origins of human culture and menstruation's effect on it. Put simply, the reason women started synchronising in the early days of homo sapiens, was in order to get the men to hunt meat because there was no need for the men to hang round as the women weren't fertile. Hence there was no point in copulation and because they weren't fertile, the women didn't need guarding from other predatory men looking to sow their seed. A sex-strike if you like.
Today there seems no obvious reason why women should synchronise. And much as some men might still leave the room at the mention of periods, there is more to most male/female sexual relationships than mere reproduction. It has also been suggested that women who synchronise have enhanced evolutionary fitness. So while we may not know fully why or how it happens, it's a little bit more complex than ''just women's things''.
Helen, 23: It's weird, I get pre-menstrual sometimes when I am not even due on. Then I find out my girlfriend is pre-menstrual as well. Then we both start on the same day. With my last girlfriend, we started coming on at exactly the same moment, and I thought that it proved we were meant for each other. Ha! Georgina, 25: I got used to having to wait around for Vicky, the girl I work closely with, to have her period so I could have mine. I came on so early one time that I had to go to the family planning clinic for advice and go straight on to another batch of pills without my seven-day break because my cycle was up the spout. The nurse was sure that I must have forgotten to take a pill and couldn't accredit it to menstrual synchrony, but I'm sure that's what did it. Suzanna, 30: Although I have lived away from home (Australia) on and off since I was 16, whenever I do spend any length of time - about two months - at home my mother and I have menstruated at the same time.
Emma, 35: A few weeks after my flatmate moved in we noticed that we were both in a PMT-induced bad temper at the same time, and both having our period at the same time. I had influenced her to fit in with me. It was quite a good thing: it was convenient to have the house like a war zone for just a few days each month, rather than both of us simmering away with PMT for twice as long. Also, we could both unite and shout twice as loud at our other [male] flatmate, if he happened to do anything sinful in the days when we were both pre-menstrual (eg fail to dry up immediately, etc).
Alex, 29: I remember trying to calm myself down one evening by having a relaxing bath and planning what to do with an unexpected baby because my period was so late (I'm normally regular). My body felt as if it was going to pop, I was suffering so badly from water retention. Then the next morning, sure enough, I came on. The next moment my flat-mate runs in saying she had just come on and it was far too early. Suddenly it all made sense. Cathy, 30: I was told about this phenomenon when I was 18 and joined the army. I thought 'what's she on about?' I didn't take any notice, but sure enough, after about five months of living with these other women I was on parade one day, two weeks away from my period, and suddenly I came on.