Background — It is generally appreciated that gestational diabetes is a risk factor for type 2 diabetes. However, the precise relation between these 2 conditions remains unknown. We sought to determine the incidence of diabetes mellitus after diagnosis of gestational diabetes.
Methods — We used a population-based database to identify all deliveries in the province of Ontario over the 7-year period from Apr. 1, 1995, to Mar. 31, 2002. We linked these births to mothers who had been given a diagnosis of gestational diabetes through another administrative database that records people with diabetes on the basis of either physician service claims or hospital admission records. We examined database records for these women from the time of delivery until Mar. 31, 2004, a total of 9 years. We determined the presence of diabetes mellitus according to a validated administrative database definition for this condition.
Results — We identified 659 164 pregnant women who had no pre-existing diabetes. Of these, 21 823 women (3.3%) had a diagnosis of gestational diabetes. The incidence of gestational diabetes rose significantly over the 9-year study period, from 3.2% in 1995 to 3.6% in 2001 (p < 0.001). The probability of diabetes developing after gestational diabetes was 3.7% at 9 months after delivery and 18.9% at 9 years after delivery. After adjustment for age, urban or rural residence, neighbourhood income quintile, whether the woman had a previous pregnancy, whether the woman had hypertension after the index delivery, and primary care level before the index delivery, the most significant risk factor for diabetes was having had gestational diabetes during the index pregnancy (hazard ratio 37.28, 95% confidence interval 34.99–40.88; p < 0.001). Age, urban residence and lower income were also important factors. When analyzed by year of delivery, the rate of development of diabetes was higher among the latest subcohort of women with gestational diabetes (delivery during 1999–2001) than among the earliest subcohort (delivery during 1995 or 1996) (16% by 4.7 years after delivery v. 16% by 9.0 years).
Interpretation — In this large population-based study, the rate of development of diabetes after gestational diabetes increased over time and was almost 20% by 9 years. This estimate should be used by clinicians to assist in their counselling of pregnant women and by policy-makers to target these women for screening and prevention.
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