As far back as three centuries ago, scientists linked higher rates of breast cancer among some women to childlessness and their inability to use their breasts as nature intended.
In the early 1700s, the Italian Bernardino Ramazzini, the father of occupational medicine, noticed that nuns in Padua had a much higher rate of breast cancer, which he attributed to their lack of child-bearing.
And research published today finally provides conclusive evidence that childbirth and breastfeeding offer substantial protection against the most common form of cancer among women in the UK.
The research was welcomed by the National Childbirth Trust (NCT) as a milestone in its 46-year campaign to promote breast-feeding, which, in public, is still widely regarded as undignified and cranky.
But today's findings, drawn from 47 studies involving 150,000 women across the world, reinforce the message that "breast is best" for both mother and child.
The research shows that the longer women breastfeed a child, and the more children they have, the more they cut the risk of developing cancer. For every year a woman breastfeeds, her risk of breast cancer goes down by 4.3 per cent, on top of a 7 per cent reduction for each child she has.
Scientists behind the research accept that it is "completely unrealistic" for today's working women to mirror mothers in rural Africa, who typically have six or seven children and breastfeed each for up to two years.
But breastfeeding a child for an extra six months could reduce a British woman's chances of developing breast cancer by 5 per cent. It is estimated that of the 39,000 cases diagnosed each year, more than 1,000 could be prevented.
British women have an average of two children and breastfeed each for two to three months. About 30 per cent do not attempt to breastfeed at all, and of every five mothers who start breastfeeding one gives up in the first two weeks.
This is a better record than the US, where 50 per cent of women do not breastfeed, but it compares badly with Scandinavia where just 10 per cent of women bottle-feed and the sight of a child on the breast hardly warrants a glance.
More women are choosing to breastfeed as the benefits become increasingly obvious. In the past decade research has shown that breastfed children score higher in intelligence tests, are less susceptible to chest infections, gastroenteritis, infections of the middle ear, asthma, eczema and diabetes and are less likely to be obese.
Along with the profound emotional benefits for mothers, breastfeeding has been shown to reduce the risk of ovarian cancer, help the uterus to contract after delivery and help women to regain their shape.
Strangely, breastfeeding was once totally acceptable in Britain. In the 1800s, it was regarded as far worse for a woman to show an ankle or leg than her breasts.
But the introduction of formula milk in the early 1900s was a turning point. At first, manufactured milk was responsible for killing nearly every baby it was fed to. Poor standards of hygiene in factories, and in the orphanages where it was used, meant that the death rate was almost 100 per cent.
But by the 1950s, formula milk had become fashionable. Advertisements showing beautiful, smiling babies coincided with an era when pregnant women rarely ventured out of the house, and when, in February 1957, a women giving birth was for the first time shown on television, the Daily Sketch declared it "tasteless" and "revolting beyond the pale".
In 1956, when the NCT was established to campaign for women to be treated more humanely during childbirth and to support mothers who wanted to breastfeed, the charity's founders were seen as freaks.
By the 1970s, Belinda Phipps, the charity's chief executive, said that few babies were breastfed. It was only during the 1980s and 1990s that breastfeeding rates began slowly to increase.
Scotland is now considering giving women a right to breastfeed, but attempts to win a right for MPs to breastfeed in the House of Commons have so far failed.
Mrs Phipps said that today's research, showing the "enormous health benefits for mothers", should encourage more women to breastfeed.
"We need to see a real shift away from the current bottle-feeding culture in the UK to one where breastfeeding is completely accepted.
"Breastfeeding should become as unremarkable as reading a newspaper, so that more women are able to follow their instincts and breastfeed wherever and whenever their baby needs to be fed.
"In Norway, nobody is interested if a mother starts to feed her baby. But we still get letters from people saying they have been thrown out of burger restaurants for breastfeeding."
She added: "We hope that this important finding – that the longer women breastfeed, they more they are protected from breast cancer – will encourage more women to consider breastfeeding."
Professor Valerie Beral, the lead author of the study, said it had long been assumed that breastfeeding and childbirth protected women. After Ramazzini published his treatise, De Morbis Artificum Diatriba, another Italian study in 1843 again showed that nuns ran a much higher risk of breast cancer.
In 1926, a scientist called Janet Lane-Claypon at the Minstry of Health found that women with breast cancer had an average of 3.4 children while women without cancer had an average of 5.3.
Professor Beral said the new study was an "important step forward" in understanding breast cancer. The key was to find the biological mechanism involved and see if scientists could replicate the protective effects of reproduction.
She said: "Prolonged breastfeeding and having lots of children pushes breast cancer rates down. It is completely unrealistic to think that there is a direct preventative message in that for women in the developed world today.
"We will have to find out how this happens and ask ourselves if there is a way of mimicking these effects in a way that is acceptable."
The recent evidence
Average intelligence test scores are higher in breastfed children
American Journal of Clinical Nutrition, 1999
Exclusive breastfeeding for at least 15 weeks is associated with a significant reduction in respiratory illness during the first seven years of life
British Medical Journal, 1998
Formula-fed babies have more urinary infections
Journal of Paediatrics, 1992
Formula-fed babies are five times more likely to be admitted to hospital with gastroenteritis in their first year than breastfed babies
British Medical Journal, 1990
Middle-ear infection is more common in formula-fed babies
Paediatrics, 1993
The introduction of milk other than breastmilk before four months is a significant risk factor for all asthma and atopy-related outcomes in six-year-old children
British Medical Journal, 1999
Eczema at the age of one and three years was lowest in babies who were breastfed for longer than six months
The Lancet, 1995
There is a higher risk of insulin-dependent diabetes in children who were formula fed
Diabetes Care, 1994
Teenagers who were fed on formula milk as babies have a higher average blood pressure
The Lancet, 2001
There is a higher risk of obesity at five to six years of age in children who were fed on formula as babies
British Medical Journal, 1999
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