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The Collaborative Group on Hormonal Factors in Breast Cancer conducts pooled analyses of data on breast cancer from 126 studies from over 30 countries worldwide. Set up in 1992, major publications include analyses on hormonal contraceptives (1996), hormone replacement therapy (1997), abortion (2004) and menarche and menopause (2012).

The first two reports of the group, on the relationships between breast cancer and use of oral contraceptives and use of hormone replacement therapy, showed that there was a small increase in the risk of breast cancer while women were using either type of hormonal therapy and in the few years after stopping. There was however, no evidence of any residual increase in risk of breast cancer many years after ceasing use of either type of therapy.

Subsequent reports have examined the influence of a wide range of factors on the risk of breast cancer. The report on familial breast cancer showed that the risk of breast cancer is most strongly related to the number of affected first degree relatives which a woman has and that the increase in the relative risk of breast cancer associated with a positive family history is greatest at young ages, and for women of a given age, is greater the younger the affected relative.

The report on breast cancer and breastfeeding showed that increasing duration of breastfeeding confers a protective effect on breast cancer risk over and above that already known to be afforded by parity itself.

The report on breast cancer risk in relation to alcohol and tobacco consumption showed that while tobacco consumption was not related to risk, there was evidence of an increase in the relative risk of breast cancer with increasing alcohol consumption.

The Group’s report on breast cancer and abortion showed that pregnancies ending in a spontaneous or induced abortion do not increase a woman’s risk of developing breast cancer.

The most recent report found that breast cancer risk is higher the younger girls are at menarche, and the older women are at menopause; but the relationship with menarche is stronger than that with menopause, suggesting that the effects are not simply explained by an increase in total reproductive years.

Future reports of the group will focus on breast cancer risk in relation to timing and pattern of childbearing, as well as height, weight and other anthropometric factors; and on risks for different cancer types.  An updated and extended report on breast cancer in relation to HRT use is also in preparation.

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