London ? The number of children women have and the length of time they breast-feed them are the most important factors influencing their chance of developing breast cancer even more important than genetic factors, major new research shows.

The landmark study, published this week in The Lancet medical journal, found that if women in the industrialized world breast-fed each of their children six months longer, they could reduce their chance of breast cancer by 5 percent, even if they have a strong family history of the disease.

Experts said the findings helped explain the mysterious rise in breast cancer rates during the last century.

“In the developed world there have been enormous changes over the last 100 years in childbearing patterns and this illustrates that those changes can explain a great deal of the increase in breast cancer rates,” said Eugina Calle, director of analytic epidemiology at the American Cancer Society.

The study involved 200 researchers across the globe examining more than 47 studies that investigated a total of 150,000 women worldwide. The analysis of the pooled information was conducted by epidemiologists at Oxford University in England.

Breast cancer rates started to climb at the end of the 19th century, and by the 1950s, it was well established that the number of children a woman had was a major factor in breast cancer.

The Oxford group started by looking at 20,000 women who had only one child and who had never breast-fed, and compared them with women who did not breast-feed but continued to have children.

“The risks go down the more children you have. Even if they’d never breast-fed, the risk of breast cancer went down by 7 percent for every additional child,” Beral said.

The researchers also found that, regardless of the number of children, the risk of breast cancer dropped by 4.3 percent for every year the women breast-fed.

The magnitude of protection was the same in all women, regardless of other characteristics, such as ethnic origin, drinking habits and age at menopause.

In the developed world, women have on average two or three children and breast-feed each for about three months.

A century ago, Western women used to have six or seven children and breast-feed each for about two years a pattern still dominant in many parts of the developing world.

Today, women in the industrialized world have a 6.3 percent chance of getting breast cancer by age 70, compared with a 2.7 percent chance for their counterparts in poor countries.

Part of the reason is that women in poor countries have children earlier, around 18 or 19, compared with 23 or 24 in the developed world.