Developing twin-specific 75-g oral glucose tolerance test diagnostic thresholds for gestational diabetes based on the risk of future maternal diabetes: a population-based cohort study

Objective — To develop twin-specific outcome-based oral glucose tolerance test (OGTT) diagnostic thresholds for GDM based on the risk of future maternal type-2 diabetes.

Design — A population-based retrospective cohort study (2007-2017).

Setting — Ontario, Canada

Methods — Nulliparous women with a live singleton (n = 55 361) or twin (n = 1308) birth who underwent testing for gestational diabetes mellitus (GDM) using a 75-g OGTT in Ontario, Canada (2007–2017). We identified the 75-g OGTT thresholds in twin pregnancies that were associated with similar incidence rates of future type-2 diabetes to those associated with the standard OGTT thresholds in singleton pregnancies.

Results — For any given 75-g OGTT value, the incidence rate of future maternal type-2 diabetes was lower for women with a twin than women with a singleton pregnancy. Using women with a negative OGTT as reference, the risk of future maternal type-2 diabetes in twin pregnancies with a positive OGTT based on the standard OGTT thresholds (9.86 per 1000 person years, adjusted hazard ratio (aHR) 4.79, 95% CI 2.69–8.51) was lower than for singleton pregnancies with a positive OGTT (18.74 per 1000 person years, aHR 8.22, 95% CI 7.38–9.16). The twin-specific OGTT fasting, 1-hour and 2-hour thresholds identified in the current study based on correlation with future maternal type-2 diabetes were 5.8 mmol/l (104 mg/dl), 11.8 mmol/l (213 mg/dl) and 10.4 mmol/l (187 mg/dl), respectively.

Conclusions — We identified potential twin-specific OGTT thresholds for GDM that are associated with a similar risk of future type-2 diabetes to that observed in women diagnosed with GDM in singleton pregnancies based on standard OGTT thresholds.