| Summary: | Objectives Though menstrual and reproductive factors have been associated with the risk of breast cancer in many populations, very few studies have been conducted among Vietnamese women. This study aimed to assess the association between menstrual and reproductive factors and the risk of breast cancer in Vietnamese women. Methods A retrospective case-control study of 490 breast cancer cases and 468 controls was conducted in Northern Vietnam. Unconditional logistic regression models adjusting for confounders were used to estimate odds ratios (ORs) and their 95% confidence intervals (CIs) for the associations of menstrual and reproductive factors with the risk of breast cancer; overall and by cancer subtype. Results Among breast cancer patients, the luminal B subtype was the most frequent (48.6%), followed by HER2-overexpressing (24.5%), luminal A (16.7%), and triple-negative breast cancer (TNBC; 10.2%). Among menopausal women, menopausal age at 50 years or older (OR = 1.71, 95% CI: 1.15-2.57 vs. <50 y) was associated with an increased risk of breast cancer. Earlier age at menarche (<13 y) was associated with a significantly increased risk of breast cancer (OR = 2.66, 95% CI: 1.08-7.51) among premenopausal women only and the luminal A subtype of breast cancer (OR = 3.06, 95% CI: 1.04-8.16). Having more than two children was associated with a reduced risk of premenopausal (OR = .42, 95%CI: .21-.83), luminal B (OR = .43, 95% CI: .24-.79), and TNBC (OR = .34, 95% CI: .14-.89). Later menopause was positively associated with the risk of breast cancer with HER2 overexpression (OR = 2.19, 95% CI: 1.14-4.23). Conclusion Associations of menstrual and reproductive factors with breast cancer among Vietnamese women, particularly for among premenopausal women and for the luminal A subtype, are generally consistent with those reported from other countries. These findings suggest that changes in menstrual and reproductive patterns among young Vietnamese women may contribute to the recent rising incidence of breast cancer in Vietnam.
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