Background: Type 2 diabetes has been associated with increased risk of gynecologic cancers, yet the effect of gestational diabetes mellitus (GDM) on gynecologic cancers is unclear.

Objectives: To examine associations between GDM history and subsequent gynecologic cancers in parous women, and to explore whether gestational hypertension (GH) plays a role in the associations.

Study design: The population-based cohort study included 15,941 individuals from the Swedish Twin Registry. The history of GDM and GH was ascertained based on self-reports. Incident cases of gynecologic cancers (including cancers of the cervix, uterus, ovaries and other female genitalia) were obtained from the National Patients Registry and the Swedish Cancer Registry. Generalized estimating equation models were applied to analyze associations between GDM and gynecologic cancers. Stratified analysis was used to explore whether associations between GDM and gynecologic cancers differed by GH. Additive and multiplicative interactions were calculated between GDM and GH.

Results: Of all participants, 350 (2.2%) had GDM, and 1762 (11.1%) had incident gynecologic cancers. No statistically significant associations were found between GDM and risks of any gynecologic cancers. However, GDM was associated with an increased risk of ovarian cancer (OR = 5.29, 95% CI: 1.63–17.19) in women with GH. Interactions between GDM and GH were observed on the additive scale (Attributable proportion due to interaction: 0.86, 95% CI 0.42–1.30, P < 0.001).

Conclusions: The associations between GDM and risks of gynecologic cancers were not evident, but the effect of GDM on the risk of ovarian cancer was modified by GH. Further validation in larger cohorts is warranted.