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Wednesday, 3 May, 2000, 17:59 GMT 18:59 UK
The fertility effect of breastfeeding
Breastfeeding emits chemicals which affect other women
Being around breastfeeding mums alters the length of other women's menstrual cycles, say researchers.

Chemicals called pheromones given off by the mums - or their babies - made the cycles significantly longer or shorter, claims Professor Martha McClintock at the University of Chicago.

Some kind of chemosignal from the breastfeeding environment is disrupting their cycles

Natasha Spencer, University of Chicago
She suggests this is due to the body attempting to maximise its chances of fertility as part of a human's desire to raise children in social surroundings.

Professor McClintock has previously carried out studies which showed women living in close proximity synchronise their menstrual cycles.

In the latest research, 26 breastfeeding women wore absorbent pads in their armpits and under their nursing bras.

Volunteers were then asked to rub the pads under their noses four times a day for two months.

The results showed that the length of women's menstrual cycles moved towards extremes after taking in the pheromones - short cycles became significantly shorter, long cycles significantly longer.


"Significantly" meant by several days, Professor McClintock said.

And she added that the time from the start of the cycle to ovulation, which is not usually shorter than five days, was less than this amount of time in a third of the women who took in the pheromones.

She told BBC News Online: "It is something that is either in the mum's sweat, or the milk or the baby's saliva.

"It is not like the women said, 'This smells of breast milk', they said it was like cotton, which the pads are made of, or like a fragrance. That is why we think they are likely to be pheromones."

Researcher Natasha Spencer added: "Some kind of chemosignal from the breastfeeding environment is disrupting their cycles."

It is something that is either in the mum's sweat, or the milk or the baby's saliva

Professor Martha McClintock
Pheromones are chemicals which are emitted by one individual, causing a physical or behavioural change in another.

Little is known about their affects on humans, though they are said to be involved in the process of sexual attraction.

Professor Bill Hansson, of the pheromone research group at Lund University in Sweden, said more evidence would be needed of the effect claimed by Professor McClintock. He said no researcher had been able to replicate the results of her previous study pointing to synchronisation of cycles.

He added: "This could be feasible but you need to be extremely cautious.

"When it comes to humans it becomes very complex because we always carry a huge amount of baggage of emotions."

Research has yet to establish whether humans have a vomeronasal organ, which some scientists claim animals use to detect pheromones.

The latest research is reported in the New Scientist.

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See also:

16 Dec 98 | Health
Putting pheromones on the map
16 Dec 98 | Health
The magic of sexual attraction
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