Women walk differently, swaying their hips to different degrees, depending on where they are in their menstrual cycle, a surprising new study concludes.

The study, conducted by researchers at Queen's University in Kingston, was published online on Thursday in Archives of Sexual Behaviour.

Previous studies have drawn connections between peak fertility and female attractiveness, suggesting that women dress more provocatively to attract mates at times when they're most likely to get pregnant.

But Queen's researchers found the opposite held true when it came to gait. They discovered that women's gaits were the least sexy when they were at optimum fertility.

Researchers recruited female first-year students and interviewed them about their use of birth control, the length of their menstrual cycle, the day of their last period and the regularity of their cycle. Only women who had regular cycles were allowed to participate in the study.

Twenty women on non-hormonal birth control, such as condoms, were selected and compared to 20 women using hormonal birth control, such as the pill or IUD. The stage of their menstrual cycle was established through saliva tests.

The women were asked to change into outfits with reflecting markers and had markers attached to their skin to allow computers to record and chart their movements. They then walked back and forth in a six-metre area.

Researchers found significant differences in gait between women at peak fertility and those at lowest fertility risk. The women who were at peak fertility, or ovulation, had narrower gaits than women whose fertile time had passed.

As well, researchers noted that there were no differences between women at high fertility risk and those on hormonal birth control. They theorize that different levels of hormones circulating in the bodies of women on birth control can change their gait on a psychological level (they have a more even mood throughout their cycle) or on a physical level (the higher levels of estrogen may loosen their ligaments and expand their gait).

The authors feel more study is needed to determine why this occurs.

As for why women walk more conservatively when they're most fertile, the Queen's researchers believe the narrower gait could be due to safety concerns on a biological level.

"The walking pattern may be a protective mechanism for the woman to avoid unwanted attention at the time of peak fertility," the authors write.