The role of menstrual and reproductive factors, family history, and body weight in the epidemiology of breast cancer has been reassessed in a meta-analysis of three large case-control studies of breast cancer from several Italian regions, for a total data set of 4,072 cases and 4,099 controls. Multiple logistic regression equations were used to obtain relative risks adjusted for study, center, age, and various combinations of risk factors considered. Relative to women with menarche at age 15 or over, those with earlier menarche had a 20-30% higher breast cancer risk. However, there was no tendency for the risk to increase with lower age at menarche, and the association with menarche was stronger at younger age. The risk of breast cancer was directly related to age at menopause (relative risk (RR) = 0.7 for less than 45 years vs. greater than or equal to 50 years), age at first live birth (RR = 1.8 for greater than or equal to 28 years vs. less than 22 years), and family history of breast cancer in first-degree relatives (RR = 2.0). The effect of these factors was similar in various age strata. After allowance for age at first live birth, the risk of breast cancer did not differ among women with one to four live births, but it was significantly below unity (RR = 0.6) for those with five or more live births. Furthermore, there was a clear modifying effect of age at diagnosis on parity-related risk, since parous women had elevated breast cancer risk below age 35 and reduced risk above age 40.(ABSTRACT TRUNCATED AT 250 WORDS)
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