Published data support the presence of etiologic heterogeneity by breast tumor subtype, but few studies have assessed this in Hispanic populations. We assessed tumor subtype prevalence and associations between reproductive factors and tumor subtypes in 1,041 women of Mexican descent enrolled in a case-only, binational breast cancer study. Multinomial logistic regression comparing HER2(+) tumors and triple-negative breast cancer (TNBC) to luminal A tumors was conducted. Compared with women with luminal A tumors, those with a later age at first pregnancy were less likely to have TNBC [OR, 0.61; 95% confidence interval (CI), 0.39-0.95], whereas those with three or more full-term pregnancies were more likely to have TNBC (OR, 1.68; 95% CI, 1.10-2.55). A lower odds of TNBC was shown for longer menstruation duration, whether before first pregnancy (OR, 0.78; 95% CI, 0.65-0.93 per 10 years) or menopause (OR, 0.79; 95% CI, 0.69-0.91 per 10 years). Patients who reported breastfeeding for more than 12 months were over twice as likely to have TNBC than luminal A tumors (OR, 2.14; 95% CI, 1.24-3.68). Associations comparing HER2(+) with luminal A tumors were weak or nonexistent except for the interval between last full-term pregnancy and breast cancer diagnosis. Findings show etiologic heterogeneity by tumor subtype in a population of Hispanic women with unique reproductive profiles. Identification of etiologically distinct breast tumor subtypes can further improve our understanding of the disease and help provide personalized prevention and treatment regimens.
