A woman's body odour can help her attract men when she is at her most fertile and repel them when she is not, scientists have said.
Men said the women's odour was most intense during menstruation
According to a report in the journal Ethology, when a woman is at the most fertile part of the menstrual cycle her armpit odour is at its mildest.
But when she is having a period, and not ready for pregnancy, the smell changes to an acute, repellent odour.
The researchers studied 12 women who wore armpit pads for 24 hours a day.
A number of primate species display changes during their fertile period, but the majority of scientists believed that this was not the case with humans.
The 12 women were restricted from eating certain foods and from using deodorant or hormonal contraceptive.
They provided odour samples on the armpit pads, which were then presented to 42 men, who sniffed them and assessed the attractiveness of the scent.
"Axillary odour from women in the follicular phase was rated as the most attractive and least intense," the study's leader Dr Jan Havlicek, from Charles University in Prague, Czech Republic, said.
"The results suggest that body odour can be used by men as a cue to the fertile period in current or prospective sexual partners," he added.
A previous study by the same team suggested that women subconsciously prefer the aroma of dominant men when they are at the most fertile stage of the menstrual cycle.
Then the researchers asked 48 men to assess how dominant they felt.
The men then wore cotton pads in their armpits for 24 hours, which were subsequently presented to 65 women.
Those who were ovulating rated the "dominant" men as sexiest, but there was no similar pattern among women at other stages of their menstrual cycle.