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Aims Breast cancer (BC) during pregnancy (PrBC) and the postpartum period (PPBC) often exhibits more aggressive tumour characteristics and is associated with a poorer prognosis compared with age-matched nonpregnant patients with BC. The underlying mechanisms for this increased aggressiveness remain unresolved. Intratumoral hypoxia, a known adverse prognostic marker in nonpregnant BC, has not yet been studied in PrBC/PPBC. This is particularly intriguing due to the potential exposure to angiogenesis-stimulating factors during pregnancy, which may influence tumour behaviour.Methods Tumour tissues from 148 patients with PrBC and 45 patients with PPBC were used to create a tissue microarray (TMA), and clinical and outcome data were obtained. The TMAs were stained for hypoxia-associated protein markers: glucose transporter-1, carbonic anhydrase IX and hypoxia-inducible factor-1α.Results Of all 193 tumours, 152 (79%) expressed at least one of these proteins indicative of intratumoral hypoxia. The presence of intratumoral hypoxia was associated with a higher histological grade (83% grade III vs 63%) and frequent hormone receptor negativity (68% vs 39%). In a multivariable analysis, the presence of intratumoral hypoxia indicated a significantly worse prognosis (HR 2.532, 95% CI 1.1 to 5.7) for patients with PrBC and PPBC.Conclusion This unique study, the first in patients with PrBC and PPBC, showed that, despite their likely exposure to angiogenesis-stimulating factors, intratumoral hypoxia is frequent and affects 79% of patients. Importantly, patients with tumours overexpressing hypoxia markers have significantly worse survival. This suggests that hypoxia may be an important mechanism in carcinogenesis and clinical behaviour of PrBC and PPBC.