Postmenopausal hormone replacement in the woman with a reproductive risk factor for breast cancer

to assess the interaction between postmenopausal hormone replacement therapy (HRT) and various reproductive risk factors for breast cancer such as early menarche, late menopause, late first delivery and nulliparity.

three cohort studies and fourteen case control studies, published between 1975 and 1997, provided relative risks (RRs) of HRT use in women with, as well as in those without, a reproductive risk factor for breast cancer.

using an additive RR model reported before, we investigated whether the RR for breast cancer in women with a combination of HRT and a given reproductive risk factor result from a simple addition of RRs of HRT on the one hand, and of the pre-existing reproductive risk factor on the other hand, or that synergism between both risk factors occurs.

simple addition of RRs was shown in the case of early menarche and late menopause. Less increase of risk, suggesting antagonism, was found for both late first delivery and nulliparity in combination with HRT use.

we could not observe any synergistic effect of the combined risks of any of the following reproductive risk factors for breast cancer: early menarche, late menopause, late first delivery or nulliparity on the one hand, with the risk resulting from HRT use on the other hand. Therefore, as far as the risk of breast cancer is concerned, the use of HRT appears not to be highly detrimental in women with a reproductive breast cancer risk factor, as it results in not more than a simple addition of risks at the most.