Prognosis of pregnancy after breast cancer diagnosis according to the type of treatment: A population-based study in Korea by the SMARTSHIP group

Backgrounds: In this study, we evaluated the incidence and outcomes of pregnancy after breast cancer was diagnosed in women of childbearing age. Additionally, we evaluated the prognosis of patients who became pregnant after breast cancer, according to the treatment. Methods: This was a retrospective cohort study of women aged 20-45 years who were surgically treated for breast cancer between 2004 and 2014 using the Korean National Health Insurance database. The patients were classified into six groups according to the treatment. Propensity score matching was applied to the cohort to analyze the risk of breast cancer-associated mortality after pregnancy and childbirth. Results: Of the 45,765 patients who had been newly diagnosed with breast cancer, 1826 (4%) became pregnant after breast cancer diagnosis. Among the pregnant group, the HR of the risk of death was 0.15 (95% CI, 0.06 to 0.36) for patients who became pregnant >49 months after the diagnosis. In patients who received endocrine therapy and chemotherapy, the pregnant group had better prognosis than the non-pregnant group. There was no significant difference between the pregnant group and the non-pregnant group in patients who received chemotherapy and trastuzumab with or without endocrine therapy. Conclusion: The risk of death was low in women who became pregnant >49 months after the diagnosis of breast cancer. The prognosis of pregnant women was non-inferior to that of non-pregnant women, even in women who received trastuzumab. These findings provide reassurance to patients with HER2-positive cancer who are considering future pregnancy.