Baby girls who are big at birth may be more likely to develop breast cancer before they reach the menopause, research suggests.

Scientists examined more than 5,000 women born in Sweden during 1915-23, of which 63 had breast cancer before the age of 50.

Exposure to high levels of growth hormones in utero may affect the developing mammary gland Valerie McCormack

They found that women who weighed 4kg (8.8lbs) at birth were 3.5 times more likely than those in women who weighed less than 3kg (6.6lbs) to develop pre-menopausal breast cancer.

Babies who were long, or who had a large head circumference seemed to be most at risk.

The researchers also found that among babies of equivalent size, breast cancer was most likely to develop in those who had a shorter period of gestation.

They believe this may indicate that the crucial factor is the rate at which a baby grows in the womb.

There was no evidence of an association between birth size and breast cancer in post-menopausal women, of which 296 had breast cancer.

Growth hormone

Researcher Valerie McCormack, from the London School of Hygiene and Tropical Medicine, said: "The mechanisms that are driving the association between the rate of foetal growth and pre-menopausal breast cancer risk are not known.

"Others have suggested that exposure to certain hormonal factors in utero may play a role.

"For instance, exposure to high levels of growth hormones in utero may affect the developing mammary gland."

The researchers stress that only a small proportion of breast cancer cases develop before the menopause, and so mothers whose babies are large should not be unduly worried by their findings.

There is research to suggest that bigger babies are less likely to go on to develop heart disease.

Professor Alan Ashworth, director of the Breakthrough Breast Cancer Research Centre, said: "This is an interesting study that may give us a better understanding about what increases the risk of breast cancer in younger women.

"The causes of breast cancer are complex. However, studies like this can help give us a better understanding of the many factors which may influence a woman's breast cancer risk.

"This could help us find new ways of predicting who is at most risk of developing the disease and give us clues about how to prevent it."

The research is published in the British Medical Journal.