By JULIE WHELDON

Last updated at 22:00 10 January 2007

Next time the woman in your life gets dressed up to the nines, you might want to take note of who she is meeting.

New research has found that women "dress to impress" when they are at their most fertile.

The discovery overturns the theory that women - unlike other female animals - always hide signs they may be ovulating.

Instead it seems women subconsciously don their trendier clothes, more jewellery, plaster on make-up and flash more flesh when they are the most fertile moment in their monthly cycle.

In the animal kingdom ovulation is often obvious, with our close relatives chimpanzees displaying swollen genitals when they are at their most fertile.

However in humans scientists have always believed it is notoriously hard to detect the time when a woman's ovaries release an egg.

Now a new study by a team at the University of California at Los Angeles has found there are clues in people after all.

The research, published in the journal Hormones and Behavior, took 30 university students aged between 18 and 37 who did not know what the study was about and were all in stable relationships.

All were photographed in their least fertile and most fertile phases.

Then a group of 42 other people - just over half of them women - were asked to look at the pairs of pictures and judge in which one the woman was trying to look more attractive.

Their faces were blacked out to make sure the assessment was based on their attire.

The judges chose the photo taken during the fertile phases 60 per cent of the time.

According to New Scientist, this is "well beyond random chance".

Lead researcher Martie Haselton said: "Near ovulation, women dress to impress, and the closer women come to ovulation, the more attention they appear to pay to their appearance.

"They tend to put on skirts instead of pants, show more skin and generally dress more fashionably."

The study's co-author April Bleske-Rechek, a psychologist from the University of Wisconsin-Eau Claire, said what was remarkable about the effect of ovulation on women was that it was so easily observed.

"In our study, the approach of ovulation had a stronger impact on the way women dressed than the onset of menstruation, which is notorious for its supposedly deleterious impact," she said.

The research builds on a new body of research showing how women's behaviour changes as they approach their most fertile period.

Previous studies have found that they are more inclined to flirt with men other than their partners and stray from their routine in ways that suggest they have a roving eye.

"Something in women's minds is tracking the ovulation cycle," said Prof Haselton.

"At some level, women 'know' when they are most fertile.

"And we have seen some evidence that men may at some level 'know' too - although with less certainty."