Childbirth in young Korean women with previously treated breast cancer: The SMARTSHIP study

Purpose: Alongside the modern trend of delaying childbirth, the high incidence of breast cancer among young women is causing significant pregnancy-related problems in Korea. We estimated the incidence of childbirth for young Korean breast cancer survivors compared with women who did not have breast cancer using a nationally representative dataset. Methods: Using a database from the National Health Insurance Service in South Korea, we analyzed 109,680 women who were between 20 and 40 years old between 2007 and 2013. They were prospectively followed, and childbirth events were recorded until December 31, 2015. We compared childbirth rates and characteristics between the breast cancer survivors and the noncancer controls. Results: Compared to 10,164 childbirths among 91,400 women without breast cancer (incidence rate: 22.3/1000), 855 childbirths occurred among 18,280 breast cancer survivors (incidence rate: 9.4/1000); the adjusted hazard ratio (HR) for childbirth was 0.41 (95% CI 0.38–0.44). Chemotherapy, endocrine therapy, and target therapy were associated with the decreasing childbirths among survivors, with corresponding adjusted HRs of 0.61 (0.53–0.70), 0.44 (0.38–0.51), and 0.62 (0.45–0.86), respectively. Breast cancer survivors had a lower probability of full-term delivery and a higher frequency of preterm labor than controls, with corresponding adjusted ORs of 0.78 (0.68–0.90) and 1.33 (1.06–1.65), respectively. Conclusions: We showed that a history of breast cancer has a negative effect on childbirth among young premenopausal women in Korea. Breast cancer survivors should be aware that they have a higher risk for preterm labor and are less likely to have a full-term delivery than women without a history of breast cancer.