Her dance moves are also a giveaway, but the way she walks provides men with the biggest clue.
Exactly what signal she gives out remains unclear - but it may involve a wiggle of the hips, the German scientists say.
More than 40 years after the George Harrison wrote the Beatles song Something, the University of Gottingen researchers set out to discover if there really is something special in the way a woman moves.
48 young women were filmed as they walked up and down in a straight line and danced for 30 second bursts.
Their movements were then turned into silhouettes, so that their appearance would not detract from their moves.
The women underwent the process twice - once when they were at their most fertile and again when the odds of them becoming pregnant were very low.
Two hundred men were then made to watch the videos of the walking or dancing silhouettes and comment on how attractive the moves were.
The results clearly showed there to be something about the women’s movements when they were at their most fertile, that was more alluring to men, a major newspaper reported.
Both their dances and walks appealed more, the women’s gait was particularly appealing around the time that her odds of pregnancy were high, the researchers reported.
The scientists didn’t scrutinise the women’s moves to find out just what was attractive to the male eye.
But researcher Bernhard Fink said that it is unlikely to do with the speed of their walk. Instead, the answer could lie in a sway, or wiggle, of the hips.
“This study suggests that women’s body movements provide subtle clues to female fertility , which men can detect and are attracted to,” he added.
The study was recently published in the journal Personality and Individual Differences.