Abstract
BACKGROUND: Although a positive association between type 2 diabetes and breast cancer has been reported, an association with gestational diabetes mellitus (GDM) is less clear. METHODS: The Sister Study enroled 50 884 women aged 35-74 years, from 2003 through 2009. Cox proportional hazards models were used to estimate breast cancer risk in relation to GDM. RESULTS: Ever having GDM was not associated with breast cancer overall (hazards ratio (HR)=1.10, 95% confidence interval (CI)=0.88-1.36), but there was a suggestive association between ever having a GDM pregnancy and oestrogen receptor (ER) -negative breast cancer (HR=1.73, 95% CI=0.98-3.06). However, having 2 or more GDM pregnancies was associated with overall breast cancer risk 1.68 (95% CI=1.15-2.44) and with ER-positive breast cancer (HR=1.81, 95% CI=1.10-2.98), which was supported by sensitivity analyses. Results were similar when analyses were stratified by whether or not type 2 diabetes had developed after GDM. CONCLUSIONS: Women with multiple GDM pregnancies had a higher incidence of breast cancer, suggesting that such women could benefit from increased surveillance for breast cancer.