Risk factors for breast cancer: a case-control study of screen-detected breast cancer in Miyagi Prefecture, Japan.

We examined the associations between reproductive factors and the risk of breast cancer on the basis of information from a total of 201,363 breast cancer screening program participants in Miyagi Prefecture, Japan, during 1987-1991. A case-control study method was applied on analysis. Data on 204 breast cancer cases identified and 810 screening year-, age- and screening area-matched normal controls were extracted. After adjustment for potential confounders, a trend of decreasing risk of breast cancer with increasing number of parity was observed (p for trend = 0.03). Among parous women, lactation for the last child decreased the risk of breast cancer (odds ratio (OR) = 0.61, 95% confidence interval (95% CI) 0.39-0.94). These findings were consistent with those in clinical breast cancer reported previously. When cases were divided into two age groups, younger (< or = 49 y.o.) and older (50 y.o. < or =), family history of breast cancer among mother and sisters (OR = 3.51, 95% CI 1.05-11.80), and lactation for the last child (OR = 0.46, 95% CI 0.25-0.84) were associated with younger age breast cancer, whereas number of parity was associated with older age breast cancer (p for trend = 0.03). The results by age group suggest that different mechanisms may exist in breast cancer developing at early and late onsets.