Breast cancer incidence rates have declined among older but not younger women; the latter are more likely to be diagnosed with breast cancers carrying a poor prognosis. Epidemiological evidence supports an increase in breast cancer incidence following pregnancy with risk elevated as much as 10 years post-partum. We investigated the association between years since last full-term pregnancy at the time of diagnosis (≤10 or >10 years) and breast tumor subtype in a case series of premenopausal Hispanic women (n = 627). Participants were recruited in the United States, Mexico, and Spain. Cases with known estrogen receptor (ER), progesterone receptor (PR), and HER2 status, with one or more full-term pregnancies ≥1 year prior to diagnosis were eligible for this analysis. Cases were classified into three tumor subtypes according to hormone receptor (HR+ = ER+ and/or PR+; HR− = ER− and PR−) expression and HER2 status: HR+/HER2−, HER2+ (regardless of HR), and triple negative breast cancer. Case-only odds ratios (ORs) and 95 % confidence intervals (CIs) were calculated for HER2+ tumors in reference to HR+/HER2− tumors. Participants were pooled in a mixed-effects logistic regression model with years since pregnancy as a fixed effect and study site as a random effect. When compared to HR+/HER2− cases, women with HER2+ tumors were more likely be diagnosed in the post-partum period of ≤10 years (OR = 1.68; 95 % CI, 1.12–2.52). The effect was present across all source populations and independent of the HR status of the HER2+ tumor. Adjusting for age at diagnosis (≤45 or >45 years) did not materially alter our results (OR = 1.78; 95 % CI, 1.08–2.93). These findings support the novel hypothesis that factors associated with the post-partum breast, possibly hormonal, are involved in the development of HER2+ tumors.