Impact of lifetime lactation on the risk and duration of frequent vasomotor symptoms: A longitudinal dose-response analysis.

OBJECTIVE: To examine the association between lifetime lactation and risk and duration of frequent vasomotor symptoms (VMS). DESIGN: Prospective cohort. SETTING: USA, 1995-2008. SAMPLE: 2356 parous midlife women in the Study of Women's Health Across the Nation. METHODS: Lifetime lactation was defined as the duration of breastfeeding across all births in months. We used generalised estimating equations to analyse risk of frequent VMS and Cox regression to analyse duration of frequent VMS in years. MAIN OUTCOME MEASURES: Frequent VMS (hot flashes and night sweats) were measured annually for 10 years, defined as occurring ≥6 days in the past 2 weeks. RESULTS: Overall, 57.1% of women reported hot flashes and 43.0% reported night sweats during follow-up. Lifetime lactation was inversely associated with hot flashes plateauing at 12 months of breastfeeding (6 months: adjusted odds ratio [AOR] 0.85, 95% confidence interval (CI) 0.75-0.96; 12 months: AOR 0.78, 95% CI 0.65-0.93) and was inversely associated with night sweats in a downward linear fashion (6 months: AOR 0.93, 95% CI 0.81-1.08; 18 months: AOR 0.82, 95% CI 0.67-1.02; 30 months: AOR 0.73, 95% CI 0.56-0.97). Lifetime lactation was associated with shorter duration of hot flashes and night sweats in a quadratic (bell-shaped) fashion. The association was strongest at 12-18 months of breastfeeding and significant for hot flashes (6 months: adjusted hazard ratio [AHR] 1.35, 95% CI 1.11-1.65; 18 months: AHR 1.54, 95% CI 1.16-2.03; 30 months: AHR 1.18, 95% CI 0.83-1.68). CONCLUSIONS: Longer lifetime lactation is associated with decreased risk and duration of frequent VMS.