Risk of Breast Cancer After Diabetes in Pregnancy: A Population-based Cohort Study.

Objectives: Diabetes is associated with an increased risk of several cancers, including postmenopausal breast cancer. The evidence for higher breast cancer risk after diabetes in pregnancy is conflicting. We compared the incidence of breast and other cancers between pregnant women with and without diabetes.

Methods: This work was a propensity-matched, retrospective cohort study using population-based health-care databases from Ontario, Canada. Those deliveries with gestational diabetes mellitus (GDM) and pregestational diabetes mellitus (pregestational DM) were identified and matched to deliveries without diabetes mellitus (non-DM). Deliveries from each diabetes cohort were matched 1:2 on age, parity, year of delivery, and propensity score to non-DM deliveries. Matched subjects were followed from delivery for incidence of breast cancer as a primary outcome, and other site-specific cancers as secondary outcomes. We performed Cox proportional hazards regression to compare rates of breast cancer between matched groups.

Results: Over a median of 8 (interquartile range 4 to 13) years of follow-up, compared with non-DM deliveries, the incidence of breast cancer was significantly lower for GDM but similar for pregestational DM deliveries (hazard ratio [HR] 0.90, 95% confidence interval [CI] 0.82 to 0.98; and HR 0.92, 95% CI 0.80 to 1.07, respectively). GDM was associated with a significantly higher incidence of pancreatic and hepatocellular cancer, and pregestational DM was associated with a higher incidence of thyroid, hepatocellular, and endometrial cancers.

Conclusions: Diabetes in pregnancy does not have a higher short-term risk of subsequent breast cancer, but there may be a higher incidence of other cancers.