As far as men are concerned, the classic giveaway that it's their partner's 'time of the month' is moody behaviour.
But new research suggests men can actually tell from a woman's voice when she is having her period.
In a study, published in the journal Ethology, psychologists asked three groups of men to listen to voice recordings of ten women who counted from one to five - at four different points over their menstrual cycle.
All four recordings were played in a random order - and then the first group of men were asked to guess which were made while the women were on their period, Popular Science reported.
The tests, conducted by Nathan Pipitone at Adams State College and Gordon Gallup, from SUNY-Albany, revealed that the men were correct 35 per cent of the time - described as a 'significant' result.
A study by the two scientists four years ago had already shown that men find the voices of ovulating women more attractive than voice recordings at other points in their period cycle.
So to update their research, they swapped the recordings which were nearest to ovulation with those from a less fertile day.
The second group in the new study still spotted the voice during menstruation 34 per cent of the time.
In fact, Pipitone and Gallup said all three group pinpointed the 'period' voices more than any other recordings.
LAPPING UP THE ATTENTION...
In another experiment, a separate group of men - who were not told the study had anything to do with menstruation - were asked to pick out the most 'unattractive' voice.
In 34 per cent of cases, they chose the menstrual recording - significantly more than was predicted by chance.
According to the men taking part in the experiments, they could tell the women were menstruating by four key indicators in the recordings: mood (bad v good); quality (harsh v smooth); pitch (low v high); and speed (slow v fast).
Some said that the menstrual recordings appeared lower in pitch, quality and mood.
In evolutionary terms, an aversion to menstruation - whether conscious or subconscious - is logical because males will seek females who are ovulating in order to pass on their genes.
Pipitone said: 'The men seemed to determine menstrual voices by picking the most unattractive voice.'
He added that it is hormones that give the women away, by inducing vocal change.
'Vocal production is closely tied to our biology. For example, cells from the larynx and vagina are very similar and show similar hormone receptors,' he said.
As a result, according to Gallup: 'The sound of a person's voice contains a surprising amount of reproductively relevant information
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