Effects of Menstrual and Reproductive Factors on the Risk of Breast Cancer: Meta‐analysis of the Case‐Control Studies in Japan

To elucidate the magnitude of the effect of menstrual and reproductive factors on breast cancer occurrence among Japanese women, we reviewed eight case‐control studies previously conducted in Japan and used a quantitative method (meta‐analysis) to summarize the data. While individual studies have different methods and populations, the estimated odds ratios (ORs) in the studies were statistically homogeneous for all study variables. It was confirmed that early age at menarche, late age at first birth, and premenopausal status are significantly associated with risk of breast cancer; an estimated combined OR of 0.68 (95% confidence interval (CI): 0.59‐0.77) was obtained for women with onset of menstruation after age 16 compared to those before age 14. Nulliparous women had higher risk than women with first birth before age 25 (OR=1.56 95%, CI: 1.27‐1.91). The OR for women with first birth after age 35 was 2.26 (95% CI: 1.85‐2.77) compared to women at first birth before age 25. Premenopausal women had a higher risk than women with menopause before age 50 (OR=2.21, 95% CI: 1.53‐3.20). We also found a significant protective effect of high parity after controlling for age at first birth and the other menstrual factors. The OR estimate for 3 or more births compared to nulliparity was 0.68 (95% CI: 0.54‐0.86). The meta‐analysis provided quantitative estimates of breast cancer risk among Japanese women with improved precision.