Comparison of age at first full-term pregnancy between women with breast cancer and women with benign breast diseases.

Benign breast diseases have a broadly similar risk profile to that of breast cancer, possibly reflecting a similar underlying endocrine milieu. We have hypothesized that a crucial distinction between breast cancer and benign breast diseases is that mammary gland terminal differentiation has not been successfully accomplished among women who tend to develop breast cancer. From October 2001 to December 2002, information concerning breast cancer risk factors and sociodemographic characteristics was collected from 174 women with breast cancer and 116 women with benign breast diseases, all 30 years old or older, who were histologically diagnosed at a major prevention center in Athens, Greece. Among the examined breast cancer risk factors, only age at first full-term pregnancy was significantly associated with the odds of having breast cancer rather than benign breast disease, and the association was evident among premenopausal [odds ratio (OR) per 5 years = 1.76, 95% confidence interval (CI) 1.10-2.93] and postmenopausal (OR = 2.10, 95% CI 1.16-3.71) women, as well as among all women (OR = 1.93, 95% CI 1.34-2.70). There was no evidence that any of the remaining breast cancer risk factors could discriminate between breast cancer and benign breast diseases. We conclude that early age at first pregnancy may convey substantial protection against breast cancer risk among women with benign breast diseases, probably operating through induction of terminal differentiation of mammary gland cells. The finding is accentuated by the fact that women with benign breast diseases are already at a relatively high risk for breast cancer. Copyright 2003 Wiley-Liss, Inc.