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A diagnosis of breast cancer during pregnancy (PrBC) does not impact prognosis if standard treatment is offered. However, caution is warranted as gestational changes in pharmacokinetics may lead to reduced chemotherapy concentration. Survival of PrBC patients treated with chemotherapy during pregnancy was compared to non-pregnant breast cancer patients treated with chemotherapy, diagnosed after 2000, excluding patients older than 45 years or with a postpartum diagnosis. The data was registered in two multicenter registries (the International Network of Cancer, Infertility and Pregnancy and the German Breast Group). Cox proportional hazards regression was used to compare disease-free (DFS) and overall survival (OS) between both groups, adjusting for age, stage, grade, hormone receptor status, human epidermal growth factor 2 status and histology, weighted by propensity scoring to account for the differences in baseline characteristics between pregnant patients and controls. In total, 662 pregnant and 2081 non-pregnant patients were selected. Pregnant patients were more likely to have stage II breast cancer (60.1% vs 56.1%, p = 0.035), grade 3 tumors (74.0% vs 62.2%, p < 0.001), hormone receptor-negative tumors (48.4% vs 34.0%, p < 0.001) or triple-negative breast cancer (38.9% vs 26.9%, p < 0.001). Median follow-up was 66 months. In multivariable analysis, DFS and OS were comparable for pregnant and non-pregnant patients (DFS: HR 1.02, 95% CI 0.82-1.27, p = 0.83; OS: HR 1.08, 95% CI 0.81-1.45, p = 0.59). Outcome of women with breast cancer treated with chemotherapy during pregnancy is comparable to young non-pregnant women. These results support chemotherapy for PrBC when indicated.

Background: A diagnosis of breast cancer during pregnancy (PrBC) does not impact prognosis if standard treatment is offered. However, caution is warranted as gestational changes in pharmacokinetics may lead to reduced chemotherapy concentration. Methods: Survival of PrBC patients treated with chemotherapy during pregnancy was compared to non-pregnant breast cancer patients treated with chemotherapy, diagnosed after 2000, excluding patients older than 45 years or with a postpartum diagnosis. The data was registered in two multicenter registries (the International Network of Cancer, Infertility and Pregnancy and the German Breast Group). Cox proportional hazards regression was used to compare disease-free (DFS) and overall survival (OS) between both groups, adjusting for age, stage, grade, hormone receptor status, human epidermal growth factor 2 status and histology, weighted by propensity scoring to account for the differences in baseline characteristics between pregnant patients and controls. Results: In total, 662 pregnant and 2081 non-pregnant patients were selected. Pregnant patients were more likely to have stage II breast cancer (60.1% vs 56.1%, p = 0.035), grade 3 tumors (74.0% vs 62.2%, p < 0.001), hormone receptor-negative tumors (48.4% vs 34.0%, p < 0.001) or triple-negative breast cancer (38.9% vs 26.9%, p < 0.001). Median follow-up was 66 months. In multivariable analysis, DFS and OS were comparable for pregnant and non-pregnant patients (DFS: HR 1.02, 95% CI 0.82–1.27, p = 0.83; OS: HR 1.08, 95% CI 0.81–1.45, p = 0.59). Conclusion: Outcome of women with breast cancer treated with chemotherapy during pregnancy is comparable to young non-pregnant women. These results support chemotherapy for PrBC when indicated.