Trimester of diagnosis affects tumor characteristics and survival in breast cancer during pregnancy: first results from the STURGATE collaboration.

Breast cancer diagnosed during pregnancy (PrBC) more often shows unfavorable disease characteristics compared to non-pregnant young breast cancer patients. Previous PrBC research suggests marked differences in tumor characteristics and survival between the gestational trimesters of diagnosis. To investigate this, data of two national retrospective PrBC cohorts from Sweden (1992-2018) and the Netherlands (1988-2022) were harmonized and pooled. PrBC patients were matched for age and year of diagnosis to three non-PrBC patients (controls) per country. Frequencies were pooled and compared using non-parametric tests, while fixed effect meta-analysis with stepwise multivariable Cox models was applied for overall survival. PrBC patients diagnosed in the first trimester resembled the non-PrBC controls. In contrast, second and third trimester diagnoses were associated with significantly more aggressive features, including higher rates of triple negative subtype, higher histologic grade, and more advanced stage at presentation compared to controls. Following adjustment for these features and treatment, overall survival remained significantly worse in PrBC patients compared to non-PrBC controls. This association was pronounced in the second and third trimesters but did not reach statistical significance after full adjustments. Altogether, the persistent worse survival for PrBC and the differences between the trimesters suggest trimester-specific biological influences on tumor behavior and survival.