Objective: To describe the clinical characteristics of breast cancer diagnosed during pregnancy and to compare their prognosis with non-pregnant breast cancer patients. Methods: This is a retrospective cohort study of 187 breast cancer patients less than 45 years of age in a single tertiary care hospital from 1995 to 2018. Clinical characteristic and outcome of 47 pregnant women who were diagnosed with breast cancer during pregnancy (pregnancy group) were reviewed, and their prognosis was compared with 140 non-pregnant breast cancer patients (non-pregnancy group). Results: The incidence of breast cancer during pregnancy was 8/10,000 (47/58,043). Termination of pregnancy was done in 4 patients, cancer treatment during pregnancy was done in 33 patients (surgery in 23 patients and chemotherapy in 32 patients), cancer treatment after iatrogenic preterm delivery was done in 8 patients, and delayed cancer treatment until delivery for obstetrical indications was done in 2 patients. The mean age of diagnosis was significantly higher in the non-pregnancy group than the pregnancy group. The stage at diagnosis, nuclear grade and Ki-67 expression rate were significantly higher, but estrogen receptor (ER) expression rate was significantly lower in the pregnancy group than the non-pregnancy group.. Other characteristics of breast cancer including histologic type, laterality, progesterone receptor (PR) expression rate and HER2 expression rate were comparable between the two groups. Mortality rate was not significantly different between the two groups (pregnancy group vs. non-pregnancy group: 17.0% vs. 7.9%, p=0.093), however, Kaplan-Meier survival analysis and Cox regression analysis showed a lower survival rates in the pregnancy group than the non-pregnancy group. Conclusion: Breast cancer during pregnancy was associated with higher stage, higher nuclear grade, higher Ki-67 expression rate and lower ER expression rate and a lower survival rate than non-pregnant controls.