Men do appear to pick up on these subtle cues. Other studies have shown that exotic dancers earn the most tips when they are closest to ovulation.

Recent studies of animals, that  showed dairy cattle and elephants change vocal utterances when most fertile, led researchers to wonder whether humans do the same.

The new study by a team in America followed 69 women for a month, using fertility tests to show when each was most and least likely to become pregnant. Voices were taped to discover the pitch and speed at which they talked.

The University of California team found that at the most fertile times of the month voice pitch rose, peaking in the two days before ovulation was due to take place.

Writing in the journal Biology Letters, Gregory Bryant, from the Department of Communication Studies, Center for Behaviour, Evolution and Culture, said the women sub-consciously added pitch when speaking to others. “This raises the intriguing possibility that cues of ovulation appear during social interactions and could serve a communicative function.”

The team now want to repeat the study, but with attractive men present during the voice tests to see whether this will increase pitch even higher.

Humans seem to have almost lost the ability to know when they are at their most fertile, leading to a boom in ovulation tests.

But an increasing body of research suggests that women may know more than they realise, albeit at a far more instinctive level.