Associations Between Duration of Lactation and Maternal Risk of Obesity-Related Cancers-A Historical Birth Cohort Study.

Lactation may protect mothers against certain cancers, such as breast- and obesity-related cancers (ORC). However, the evidence is limited and has methodological shortcomings. Therefore, we investigated the associations between lactation durations and the maternal risk of ORC as well as endometrial, ovarian, and postmenopausal breast cancer individually. We included 6880 mothers from the Copenhagen Perinatal Cohort giving birth between 1959 and 1961. Information on duration of predominant and any lactation was collected 1 year postpartum. We used the International Agency for Research on Cancer definition of ORC, including 13 cancers. Cancer diagnoses were obtained from The Danish Cancer Registry (nORC = 1300, nendometrial = 97, novarian = 89, npostmenopausal breast = 591) during 1968-2022. Hazard ratios (HR) and 95% confidence intervals (CI) for the cancer risk were estimated using Cox regressions adjusted for demographic, metabolic, and reproductive factors. Duration of any lactation was not associated with the risk of either ORC (HR = 1.00 [95% CI: 0.96-1.03] per month), or endometrial (0.95 [0.84-1.08] per month), ovarian (0.94 [0.82-1.09] per month) or postmenopausal breast (0.98 [0.94-1.03] per month) cancer individually. When we investigated the duration of predominant lactation, we found similar results. In this historical birth cohort study with 60 years of follow-up, lactation was not associated with a clinically relevant reduction in maternal risk of ORC or of endometrial, ovarian or postmenopausal breast cancer individually. Prospective studies with longer durations and varying intensities of lactations are needed to elucidate potential associations more completely. Importantly, the lack of any directional findings supports leaving breastfeeding recommendations in place.