Whether pregnancy-associated breast cancer (PABC) arise before or during pregnancy and whether this histopathology is affected by gestational age are currently unclear. The present study assesses the influence of gestational age and lactation on the histopathologic profile of PABC. We identified 744 patients with PABC (defined as breast cancer during pregnancy or 6 months following delivery). Histopathologic features were compared between pregnant and postpartum patients. We found that age at diagnosis was 34.2 years, and a majority of cancers were diagnosed during pregnancy (71.3%). Within pregnant patients, tumors were significantly more often estrogen receptor (ER)-negative in second and third trimesters (57.4%), as compared to first trimesters (41.9%) (P = 0.036). Similarly, a progesterone receptor (PR)-negative status was reported significantly less often within first trimesters (38.0%) compared to second and third trimesters (57.1%) (P = 0.032). For human EGF receptor 2 status, no significant differences were observed between gestational trimesters or lactating vs non-lactating patients. In postpartum patients, grade III tumors were found in over 80%, with high percentages of ER-negative tumors reaching 63% in those lactating vs 49% in non-lactating patients. This study demonstrates the varying histopathologic profile of PABC by gestational age and lactation status. Second- and third-trimester cancers display most typically the common ER/PR-negative phenotype, which is commonly reported in the literature. The increased ER-negative status and percentage grade III tumors in lactating vs non-lactating patients also suggest the presence of additional factors further diversifying histology. This indicates the need for clear definitions of PABC and the role of potential subgroups, which may provide a stepping stone for further in-depth research into PABC-carcinogenesis.