Diabetes mellitus following gestational diabetes: role of subsequent pregnancy

Objective
This study was designed to determine the rate of diabetes up to 13 years after pregnancies complicated by gestational diabetes and to identify risk factors for developing diabetes. The role of a subsequent pregnancy, with and without gestational diabetes, was also examined.

Design
This was a retrospective cohort study of women with gestational diabetes.

Population and setting
Women who had gestational diabetes in their first pregnancy between 1989 and 2002 were identified through a population-based perinatal database in Nova Scotia, Canada.

Methods
Subsequent diagnoses of diabetes, up to 13 years after the first pregnancy, were obtained from physician billing and hospital discharge databases. Cox proportional hazards regression models were used to estimate adjusted relative risks (RR) and 95% confidence intervals.

Main outcome measures
Diagnosis of diabetes after pregnancy.

Results
Of the 1401 nulliparous women with gestational diabetes, 251 women (17.9%) developed diabetes in the follow-up period. The cumulative incidence at 1, 5, and 10 years was 5.9, 14.8, and 22.2%, respectively. Factors significantly associated with an increased risk of developing diabetes mellitus included a pre-pregnancy weight of ?86 kg (RR = 1.8, 95% CI 1.2–2.9), insulin therapy during the index pregnancy (RR = 4.1, 95% CI 2.1–7.9), neonatal hypoglycaemia (RR = 2.6, 95% CI 1.6–4.2), and a subsequent pregnancy with gestational diabetes (RR = 2.3, 95% CI 1.6–3.4).

Conclusion
Indicators of the severity of gestational diabetes, defined by insulin use, neonatal hypoglycaemia, and recurrent gestational diabetes in a subsequent pregnancy, are important in predicting a subsequent diagnosis of diabetes. Our findings do not support the theory that subsequent pregnancy, per se, increases the risk of developing diabetes.

Introduction

Gestational diabetes mellitus (GDM) is defined as ‘carbohydrate intolerance of varying degrees of severity with onset or first recognition during pregnancy’. The prevalence of GDM in Canada is approximately 3.5% in the non-Aboriginal population. The recurrence rate for GDM in subsequent pregnancies has been estimated at 36% in the Nova Scotia population. GDM is associated with an increased risk of subsequent type II diabetes mellitus (DM). In the general female population of Nova Scotia in 2002/03, the prevalence of diabetes is 0.86% for women of age 20–29 years, 1.74% for women of age 30–39 years, and 3.67% for women of age 40–49 years. These rates are slightly higher than prevalence rates among white women less than 44 years of age in the USA.

Cumulative incidence rates of DM among women with a history of GDM vary widely depending on the length of follow up and the underlying risk of DM in the population. Among women with a history of gestational diabetes, it is generally accepted that race, age, parity, family history of diabetes, pre-pregnancy weight, postpartum obesity, and weight gain are risk factors for developing DM. Other suspected risk factors include smoking, physical inactivity, diet, and drugs that adversely affect glucose metabolism. It has been suggested that increased insulin resistance associated with pregnancy may accelerate the depletion of beta cells.5 If this is the case, it would be expected that increased parity or subsequent pregnancies would increase the risk for development of subsequent diabetes among women with a history of gestational diabetes. However, findings related to the roles of parity and subsequent pregnancies are conflicting.