LAUSR

Few risk factors have been identified for triple negative breast cancer (TNBC) which lacks expression of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2). This more aggressive subtype disproportionately affects some racial/ethnic minorities and is associated with lower survival. We pooled data from three population‐based studies (558 TNBC and 5,111 controls) and examined associations of TNBC risk with reproductive history and breast‐feeding. We estimated odds ratios (ORs) and 95% confidence intervals (CIs) using multivariable logistic regression. For younger women, aged <50 years, TNBC risk was increased two‐fold for parous women who never breast‐fed compared to nulliparous women (OR = 2.02, 95% CI = 1.12–3.63). For younger parous women, longer duration of lifetime breast‐feeding was associated with a borderline reduced risk (≥24 vs. 0 months: OR = 0.52, 95% CI = 0.26–1.04, Ptrend = 0.06). Considering the joint effect of parity and breast‐feeding, risk was increased two‐fold for women with ≥3 full‐term pregnancies (FTPs) and no or short‐term (<12 months) breast‐feeding compared to women with 1–2 FTPs and breast‐feeding ≥12 months (OR = 2.56, 95% CI = 1.22–5.35). None of these associations were observed among older women (≥50 years). Differences in reproductive patterns possibly contribute to the ethnic differences in TNBC incidence. Among controls aged <50 years, the prevalence of no or short‐term breast‐feeding and ≥3 FTPs was highest for Hispanics (22%), followed by African Americans (18%), Asian Americans (15%) and non‐Hispanic whites (6%). Breast‐feeding is a modifiable behavioral factor that may lower TNBC risk and mitigate the effect of FTPs in women under age 50 years.