By FIONA MACRAE

Last updated at 23:24 17 October 2007

It is widely thought to give a baby the best start in life.

But breastfeeding may have another, more unexpected, benefit.

Smells associated with breastfeeding are a natural aphrodisiac, heightening sexual desire in other women, a study has found.

Researchers discovered exposure to the subtle odours produced by breastfeeding mothers and their babies can raise libido by almost 50 per cent.

Speaking at the American Society for Reproductive Medicine's annual conference in Washington, the psychologists behind the study said they believe the smells - known as pheromones - act as a signal, telling a woman it is time to try for a baby.

They hope to use these chemicals to develop treatments to help women who suffer from a low sex drive.

The researchers from the University of Chicago looked at how the smell of sweat collected from the breast and armpits of nursing mothers affected a group of young women.

Half were asked to wipe pads soaked in the sweat across their upper lip every morning and evening for three months.

They were also asked to reapply the pads after showering, exercising and wiping their mouths after eating. The remaining half were given similar pads, soaked in a dummy liquid.

Neither group knew what was on the pads.

By the end of the study, those given the sweat-soaked pads found their desire for their partner had risen by around 42 per cent, while those who were single had more sexual daydreams than usual.

The effects were particularly striking during the time of the month when the women's fertility and sex drive was at its lowest. In contrast, libido actually dropped among those with dummy pads.

It is thought those given the sweat-soaked pads were affected by the pheromones, which are secreted by the body to affect behaviour, including sexual attraction, on an unconscious level.

In this case, it is thought they acted as a signal to other women that food is plentiful and the environment is safe to bring a baby into.

Having a baby at the same time as other women would also allow the burden of childcare to be shared.

If the key compounds in the sweat, breast milk and baby's saliva could be identified and bottled, they could be turned into a powerful aphrodisiac for women, the study added.

Researcher Professor Martha McClintock, said: "We found that being exposed to these chemicals sustained sexual desire during times in the cycle when it would normally be lower than other peak levels.

"It could be used for the treatment of disorders of desire. A lot of basic research would need to be done to identify the specific compounds involved.

"For men the major problem is erectile dysfunction (impotence) for which there is Viagra.

"But for women it is a disorder of desire and there isn't anything as effective."

Scientists also believe sweat may play a vital part in the mating game because we seek out partners whose body odour is different from our own.

It is thought the phenomenon evolved to prevent us unintentionally mating with relatives or seeking a partner who is genetically similar.