Pre- and perinatal factors and incidence of breast cancer in the Black Women’s Health Study

PURPOSE: To investigate the association between pre- or perinatal factors and breast cancer risk among African American women. METHODS: Participants in the Black Women’s Health Study, a prospective cohort of 59,000 African American women, reported birth weight, preterm birth, twin or triplet status, maternal age at birth, birth order, and having been breastfed during infancy at various times during follow-up from 1997 to 2015. Numbers of incident cases ranged from 312 for breastfed analyses to 1,583 for twin or triplet analyses. Using multivariable Cox proportional hazards regression, we estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for associations between each factor and breast cancer risk overall and by estrogen receptor (ER) status. RESULTS: Compared to birth weights of 5 lbs. 8 oz.−8 lbs. 13 oz., low (8 lbs. 13 oz.) birth weights were associated with increased breast cancer risk; HRs (95% CI) were 1.19 (0.98–1.44) and 1.26 (0.97–1.63), respectively. Associations were similar by ER status. Having been born to a mother aged ≥35 years vs.