Key clinical point: Use of female hormones was associated with an increased incidence of breast cancer (BC) in premenopausal women. Among postmenopausal women, older age at first birth increased the incidence of BC, whereas more births decreased it.
Major finding: Use of female hormones significantly increased the incidence of premenopausal breast cancer (adjusted hazard ratio [aHR], 1.53; P = .03). Incidence of postmenopausal breast cancer was higher in women aged 26 years or more vs. 21-25 years at first birth (aHR, 1.34, 1.47, and 1.68 for ages 26-30, 31-35, and 36 years and older, respectively; P less than .001) and lower in women giving more vs. no births (aHR, 0.85, 0.65, and 0.52 for 1, 2, and 3 and more births, respectively; P = .02).
Study details : Findings are from a pooled analysis of 9 population-based cohort studies conducted in Japan including 187,999 women (premenopausal, n=61,113; postmenopausal, n=126,886).
Disclosures: This study was supported by the National Cancer Center Research and Development Fund and the Health and Labour Sciences Research Grants for the Third Term Comprehensive Control Research for Cancer. The authors had no disclosures.
Source: Takeuchi T et al. Cancer Med. 2021 Mar 1. doi: 10.1002/cam4.3752 .