Introduction: Pregnancy rarely coincides with breast cancer, but when it does, uncertainties remain about how survival is affected. In a nation-wide study, we investigated survival in women diagnosed with breast cancer during pregnancy. Materials and Methods: Through health registries, we identified women with breast cancer at ages 15-44 years from 1973-2016 in Denmark and included 156 who were pregnant at diagnosis and 11,110 who were not. We compared overall mortality in pregnant and non–pregnant women using multivariate Cox regression stratified by time since cancer: <2 and ≥2 years. Results: During the first 2 years after diagnosis, the hazard ratio of overall death was 2.28 (95% CI: 1.48-3.52) for pregnant versus non–pregnant breast cancer patients after adjustment for age and calendar period and 1.62 (95% CI: 1.05-2.50) after further adjustment for extent of disease. Adjusting for additional tumor characteristics, the hazard ratio was still significantly increased. Beyond the first 2 years, there was no excess mortality. Conclusion: Our study identifies the early period after breast cancer as a period of particular interest in future studies on survival after breast cancer in pregnancy. We found no evidence that survival is affected by pregnancy when 2 or more years have passed since diagnosis.