The risk of developing type 2 diabetes after gestational diabetes: A registry study from Finland

Aims: Women with a history of gestational diabetes (GDM) have an increased risk of developing type 2 diabetes (T2DM). We studied the risk for T2DM in women with and without GDM in relation to body mass index (BMI) and examined whether insulin treatment for GDM associates with the risk of developing T2DM. In addition, we investigated whether the risk of developing T2DM after GDM had changed in 15 years. Methods: We used data by linking four registers; Medical Birth Register, Hospital Discharge Register and Primary Care Register run by THL Finnish Institute for Health and Welfare, and Medical Reimbursement Statistics run by the Social Insurance Institution of Finland (Kela). Registry data were collected from 2005 to 2020. The follow-up started from woman's delivery in 2006-2020 and ended to the diagnosis of T2DM or December 2020. Cox proportional hazard modelling was used to estimate the effect of GDM exposure to T2DM. To assess whether the risk of developing T2DM after GDM had changed in 15 years, we compared the HR between years 2006-2008 and 2018-2020. Results: In total, 462 401 women were included in the study: 96 353 (21%) women had previous GDM. There were 5370 (1.2%) women who developed T2DM after childbirth during the follow-up. Among women with prior GDM, 3995 (4.1%) developed T2DM, while 1375 (0.4%) women without prior GDM developed T2DM during follow-up. The mean follow-up was 6.86 years (SD 4.21) for women with GDM and 9.07 years (SD 4.35) for women without GDM. The hazard ratio (HR) for developing T2DM after GDM was 18.49 (95% CI 17.39-19.67). The incidence of T2DM in women with a history of GDM began to rise almost steadily from the first year of follow-up. As BMI increased, T2DM incidence increased in both women with and without prior GDM but more in women with prior GDM. Insulin treatment had an independent association with increased risk of T2DM (HR 3.81, 95% CI 3.57-4.07). We did not observe any difference in HR between years 2006-2008 and 2018-2020. Conclusions: The relative risk for T2DM was 11-fold for women with previous GDM compared to women without previous GDM. A higher BMI and insulin treatment increased the risk of future diabetes. All measures to prevent the conversion of GDM to T2DM should be taken especially among women with overweight or obesity.