Sydney - Many women Olympic athletes go on the contraceptive pill over worries that their period will fall on the day of a critical event and blight their chances of winning a medal.

For all but distance runners in the second half of their menstrual cycle competing on a hot day, their worries are misplaced, an Australian researcher said.

“There've been gold medals won and world records set at each stage of the menstrual cycle, so it's not as if there's clear evidence there saying you can't win a medal when you're having your period,” Newcastle University's Xanne Janse de Jonge said.

And it could be that further research into hormonal changes will allow athletes to harness the menstrual cycle to actually improve performance because early work shows that they may be stronger nine to 13 days after the start of bleeding.

“The easiest way to split it up is the first two weeks and the second two weeks,” de Jonge said. “The second two weeks of the monthly cycle are when the temperature is higher and that's definitely where temperature regulation is a problem.”

But only a big problem for marathon runners and only on hot days, because temperature only rises half a degree and an inbuilt mechanism raises the body's responses by half a degree as well.

“So they can still control their temperature at that new set point, which is half a degree higher,” she said.

The Dutch-born researcher said much more work needed to be done before the performance effects of the pill were understood.

“The hard thing there is that we know even less about what's happening there than we do on the normal menstrual cycle,” she said. “There's just not enough information out there to give that advice. We haven't actually worked out what's happening.”

Many of the female athletes in London will not have regular monthly cycles.

“The heavy training might disrupt the cycle,” she said. “It's the body saying that I'm under so much stress that it wouldn't be safe for me to bring a baby into this environment. The body shuts down. It's a normal inbuilt mechanism.”

Another question about hormones that the Olympics often throws up: Are the boyish figures of 15-year-old gymnasts evidence of exercise delaying the onset of puberty?

“I wouldn't say it delays the onset,” de Jonge said. “The athletes who are participating in sports where it's beneficial for them not to be mature usually continue in those sports. Those who mature quite early would drop out because their physical shape starts changing.” - Sapa-dpa