A new study found that big babies may have a greater risk of developing breast cancer before age 50.

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A study in this week's British Medical Journal found a link between birth size and risk of premenopausal breast cancer.

More than 5,000 women born in Sweden from 1915 to 1929 were included in the study, of which 63 had breast cancer before the age of 50.

Researchers found a strong association between larger birth size and rates of breast cancer at premenopausal ages, even when other risk factors were taken into account. Birth length and head circumference had stronger links with early breast cancer than birth weight.

They also found that a shorter period of pregnancy was also linked to an increased risk of breast cancer before the age of 50, indicating that "the rate of fetal growth may underlie the association between birth size and risk of early breast cancer" the authors suggested.

There was no evidence of an association between birth size and breast cancer in postmenopausal women.

However, the researchers said that if the findings prove true, large birth size would be responsible for only a small proportion of the total number of cases of breast cancer since the incidence at premenopausal ages is low.

They also said this association should be considered in light of its opposite association with heart disease, a much more common condition, they said.