Breastfeeding history and adenomyosis risk using a novel case-control study design.

Objective: To evaluate the association between breastfeeding history, including lifetime exclusive breastfeeding, and risk of adenomyosis.

Design: We used data from a case-control study designed with 2 control groups to address the challenge of selecting noncases for a valid epidemiologic study when cases are identified by hysterectomy. The case-control study was conducted among premenopausal and postmenopausal enrollees aged 18-59 years in a large, integrated health care system in western Washington state.

Patient(s): Cases were enrollees with incident, pathology-confirmed adenomyosis diagnosed during 2001-2006 (n = 386). The 2 control groups were as follows: (1) randomly selected age-matched enrollees with intact uteri ("population controls," n = 323) and (2) hysterectomy controls (n = 233).

Intervention(s): Data on breastfeeding history were collected by in-person interviews. For each reported live birth, participants were asked whether they breastfed, along with infant age at supplemental feeding introduction and breastfeeding discontinuation.

Main Outcome Measure(s): Among participants with at least 1 live birth (330 cases, 246 population controls, and 198 hysterectomy controls), we used unconditional logistic regression to estimate adjusted odds ratios and 95% confidence intervals (CIs) for the associations between the following: (1) ever breastfeeding, (2) ever breastfeeding for ≥8 weeks, (3) lifetime breastfeeding, and (4) lifetime exclusive breastfeeding and risk of adenomyosis. Analyses were adjusted for age, reference year, smoking, education, and parity.

Result(s): In analyses comparing cases with population controls, we observed a 40% decreased odds of adenomyosis with a history of ever breastfeeding (adjusted odds ratio, 0.6; 95% CI, 0.3-1.0) and breastfeeding for ≥8 weeks (adjusted odds ratio, 0.6; 95% CI, 0.4-0.8). The strongest associations, 60%-70% decreased odds of adenomyosis, were observed with ≥12 months of lifetime breastfeeding (vs. <3 months) (adjusted odds ratio, 0.4; 95% CI, 0.2-0.6) and 9 to <12 months of lifetime exclusive breastfeeding (vs. <3 months) (adjusted odds ratio, 0.3; 95% CI, 0.2-0.6), comparing cases to population controls. In analyses using hysterectomy controls, we observed similar patterns of associations slightly attenuated in magnitude.

Conclusion(s): Breastfeeding history was associated with a 40% decreased odds of adenomyosis, a condition that can confer substantial morbidity and requires hysterectomy for definitive treatment. The consistency of our findings with that of a previous study lends support that breastfeeding may modify risk of adenomyosis.