BACKGROUND Pregnancy concurrent with, shortly before, or after breast cancer poses unique challenges because hormonal changes in pregnancy potentially interact with breast cancer outcomes.

MATERIALS AND METHODS We studied a cohort of 3,687 female patients of reproductive age (<50 years) with breast cancer, linking a large institutional database and the nationwide claims database to comprehensively capture exposure status and tumor characteristics. Exposures included breast cancer during pregnancy, postpartum breast cancer (<12 months after delivery), and pregnancy after breast cancer.

RESULTS Forty-five patients with postpartum breast cancer were significantly more likely to have advanced stage, hormone receptor-negative tumor and to be younger than 35 years at diagnosis than those without postpartum breast cancer. This trend was not observed with 18 patients with breast cancer during pregnancy. The unadjusted 5-year survival rates were 77% versus 96% for patients with postpartum breast cancer versus their counterparts, 89% versus 96% for patients with breast cancer during pregnancy versus their counterparts, and 98% versus 96% for patients with pregnancy after breast cancer versus their counterparts, respectively. In the multivariable analyses, postpartum breast cancer exhibited hazard ratios for death of 1.57 (95% confidence interval [CI], 0.82-2.99), whereas those for breast cancer during pregnancy and pregnancy after breast cancer were 1.09 (95% CI, 0.15-7.91) and 0.86 (95% CI, 0.26-2.83), respectively.

CONCLUSION Postpartum breast cancer, but not breast cancer during pregnancy, was associated with advanced stage, younger age at diagnosis (<35 years), hormone receptor-negative disease, and poorer survival. Pregnancy after breast cancer did not compromise overall survival.

IMPLICATIONS FOR PRACTICE Although pregnancy around the time of diagnosis of breast cancer is expected to become increasingly common with maternal age at first childbirth on the rise, data on the prognostic impact of pregnancy have been inconsistent and rare from Asian populations. In this investigation of a Korean patient cohort with breast cancer, pregnancy-associated breast cancer was associated with advanced stage, younger age at diagnosis (<35 years), hormone receptor-negative disease, and poorer survival. This adverse impact of pregnancy on the prognosis was apparent with postpartum breast cancer but not observed with breast cancer during pregnancy. Pregnancy after breast cancer did not compromise overall survival.