Although a positive association between type 2 diabetes and breast cancer has been reported, an association with gestational diabetes mellitus (GDM) is less clear.

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.

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.

Women with multiple GDM pregnancies had a higher incidence of breast cancer, suggesting that such women could benefit from increased surveillance for breast cancer.British Journal of Cancer advance online publication, 16 February 2017 doi:10.1038/bjc.2017.34 www.bjcancer.com.