Sportswomen are more prone to injuries at certain times during their menstrual cycle, a study paper suggests.
The study looked at female football players
The Swedish research, by a PHD student at Luleň University, included 30 of the nation's top women's soccer teams.
Of the 319 female players studied, half received injuries, most commonly to the ankle, knee and thigh.
Compared with team mates on the oral contraceptive pill, women were more injury-prone when they had their period than when they were not menstruating.
It is not clear exactly why this is, but researcher Inger Jacobson believes it might be related to hormonal fluctuations during the menstrual cycle.
For example, the level of a hormone called relaxin, which relaxes or slackens the ligaments, goes up around menstruation.
Other studies have drawn similar conclusions.
Published work has shown women perform worse on a skill called joint position sense - judging the position of a limb joint, such as the extent to which the knee is bent (flexed) or straight (extended) - around the time of their period.
Coordination, postural control, reaction time and judgement may also be affected, experts believe.
Oestrogen is known to affect pain perception, thus a sportswoman might be more likely to report injuries during low-oestrogen states, such as around the time of her period, suggests Ms Jacobson.
Overall, both groups - female soccer players on the pill and those not - had similar rates of injury.
John Brewer, director of the Lucozade Sport Science Academy, said: "The current doctoring is actually the opposite - when female athletes have no periods they are at increased risk of injury because they have a lower bone mineral density and that greatly increases the risk of stress fractures and bone problems."
He said it made sense that relaxin levels might explain the latest study's findings.
"If your tendons and ligaments become more flexible then they are more actively overstretched and cause damage to the joints and ligaments themselves," he explained.
He said in the latter stages of pregnancy women are advised against rigorous sports that involve a great deal of twisting and turning for this reason.
Dr Pippa Bennett, the FA women's team doctor, said it was recognised that there were phases in a woman's menstrual cycle that could affect sporting performance.
There can be a slight tail off in things like coordination just before menstruation, and when hormone levels peak ligaments can become more lax, she said.
"Certainly, with the England women's team that is something that we have been keeping an eye on, tracking the various injuries that our players get."
But she said it was not possible to say that a player would definitely be at a higher risk of injury at certain times of the month.