Adenomyosis is characterized by presence of endometrial tissue within the muscular wall of the uterus and is associated with substantial morbidity. While etiology of adenomyosis remains unknown, an estrogenic milieu contributes to disease pathogenesis. We hypothesize that lactation, wherein infant suckling inhibits ovulation and induces a hypoestrogenic state, is associated with decreased adenomyosis risk. We investigated this hypothesis using data from a case-control study of adenomyosis conducted among female enrollees of a large healthcare system in Washington State. In that study, incident, pathology-confirmed adenomyosis cases diagnosed 2001-2006 were identified and two control groups were employed: randomly selected age-matched enrollees with intact uteri (“population controls”) and hysterectomy controls. Breastfeeding history of initiation and duration for each live birth reported were collected by in-person interview. We restricted the analytic sample to those with at least one live birth (330 cases, 246 population controls, and 198 hysterectomy controls) and used logistic regression to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for associations between adenomyosis and breastfeeding, adjusting for age, reference year, smoking, education, and parity. Using population controls, history of ever breastfeeding or ever breastfeeding an infant for ≥ eight weeks were associated with a 40% decreased risk of adenomyosis (ever breastfed: OR 0.6, 95% CI: 0.3, 1.0; ever breastfed an infant ≥ eight weeks: OR 0.6, 95% CI: 0.4, 0.8). The magnitude of association was stronger with longer lifetime breastfeeding duration (≥12 months vs. 0-<3 months: OR 0.4, 95% CI: 0.2, 0.6) and exclusive breastfeeding (≥12 months vs. 0-<3 months: OR 0.4, 95% CI: 0.2, 0.6). Using hysterectomy controls, we observed similar patterns of associations that were attenuated in magnitude. Our results indicate that a potentially modifiable factor, breastfeeding, may decrease adenomyosis risk among parous women.
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