Reproductive factors and breast cancer in New Zealand

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Abstract
A national population‐based case‐control study was used to assess the influence on breast cancer risk of reproductive factors and the possibility of an interaction with age at diagnosis. A total of 891 women aged 25 to 54 with a first diagnosis of breast cancer, and 1864 control subjects, randomly selected from the electoral rolls, were interviewed. There was a declining risk of breast cancer with increasing age at menarche (p = 0.06), the strongest effect being seen in women aged less than 40. Parous women had a 27% lower risk of breast cancer than nulliparous women, a reduced risk being evident in all but the youngest age group. A falling risk of breast cancer with rising parity was clear only in women diagnosed when aged at least 45 years. Breast cancer risk tended to fall amongst parous women with increasing duration of breastfeeding (p = 0.14); the association was most apparent in the youngest women, while those over 40 years at diagnosis showed no clear negative trend. There was no association of breast cancer risk with age at first full‐term pregnancy, time since last full‐term pregnancy, abortion (spontaneous or induced), abortion before first full‐term pregnancy, or ability to conceive, and there was no trend in risk with age at natural menopause. Women in the highest category of body mass index at age 20 had the lowest risk of breast cancer in the age group studied. When each reproductive factor was formally tested for effect modification by age at diagnosis, the interaction term in logistic models approached statistical significance only for parity (p = 0.07). Int. J. Cancer 76:182–188, 1998.