Converging evidence indicates that women with pregnancy-associated breast cancer (PABC) have increased mortality compared to women with breast cancer not diagnosed near pregnancy (non-PABC). Our aim was to investigate if the stage distribution differs between PABC and non-PABC and if stage at diagnosis can explain the poorer prognosis observed among women with PABC. We identified 3,282 breast cancers in women aged 15-44 years at diagnosis for whom staging data (tumor size, nodal involvement, metastasis) were available in the Swedish Cancer Register between 2002 and 2009. Information on reproductive history and vital status was obtained from the Multi-Generation Register and the Cause of Death Register. PABC was defined as breast cancers diagnosed during pregnancy and up to 2 years after delivery (n = 317). Non-PABC was defined as cases diagnosed before pregnancy or more than 2 years postpartum. Stage distributions were compared between PABC and non-PABC, and mortality rates were modeled using Cox regression. Compared to women with non-PABC, the mortality was almost 50 % higher in women with PABC [unadjusted hazard ratio (HR) 1.47 (95 % CI 1.04-2.08)], a difference which was reduced after adjustment for age and calendar year of diagnosis [HR 1.27 (95 % CI 0.88-1.83)]. Although advanced stage of breast cancer at diagnosis was more common among PABC than among non-PABC, further adjustment for stage only slightly reduced the HR [1.22 (95 % CI 0.84-1.78)]. The difference in mortality between PABC and non-PABC was more pronounced among women above 35 years and among women with PABC diagnosed within 1 year postpartum. Age, rather than stage at diagnosis, appears to act as the principal driver of the increased mortality observed in women with PABC. However, these findings do not preclude an untoward influence on mortality by pregnancy-associated factors affecting tumor aggressiveness and progression.