Objective: Several studies have found inverse associations between breastfeeding and ovarian cancer occurrence but there are inconsistencies. The relationship with breastfeeding duration is unclear, and it is uncertain whether the association varies according to histological subtype of ovarian cancer. We sought to clarify these issues.
Methods: Parous women who participated in an Australia-wide population-based case-control study of epithelial ovarian cancer between 2001 and 2005 (1,092 cases and 1,288 controls) completed a reproductive/lifestyle questionnaire. Logistic regression was used to calculate odds ratios and 95% confidence intervals to investigate relations between breastfeeding and ovarian cancer.
Results: We found a strong inverse association between total duration of breastfeeding (all episodes combined) and ovarian cancer occurrence (OR = 0.986, 95% CI 0.978-0.994 per month of breastfeeding) that appeared to be independent of parity. However, individual episodes of lactation beyond 12 months conferred no appreciable additional benefit. The relation with breastfeeding appeared to vary by histological subtype such that we saw no association between duration of breastfeeding and borderline serous cancers or mucinous cancers, but inverse associations for the other subtypes, although these were not always statistically significant.
Conclusions: A long total duration of breastfeeding appears to be associated with a substantial reduction in the overall risk of ovarian cancer, independent of the decrease in risk due to childbirth, but this may vary according to histological subtype.