Screening for Diabetes in Pregnancy

Gestational diabetes mellitus (GDM), defined by the World Health Organization (WHO) as ‘carbohydrate intolerance resulting in hyperglycaemia of variable severity with onset or first recognition during pregnancy’1, is the most common metabolic disorder of pregnancy. It is evident that GDM defined in this way will include patients with undiagnosed preexisting type 2 diabetes mellitus (type 2 DM) as well as those with diabetes with first onset during pregnancy. Normal pregnancy is associated with insulin resistance similar to that found in type 2 DM. The reduction in whole-body insulin action becomes apparent in the second trimester, and insulin sensitivity declines progressively to term2,3. After delivery, whole-body insulin action returns rapidly to normal4. Hence, if the onset of diabetes is truly in pregnancy, then it is most likely to occur in the second and third trimesters and glucose tolerance may return to normal after delivery. Nonetheless, women with GDM in whom glucose tolerance becomes normal post-partum remain insulin-resistant compared with women with no history of GDM5. Diabetes in the first trimester is more likely to be type 2 DM that was present but undiagnosed before pregnancy. In these circumstances, glycosylated haemoglobin, giving an indication of average blood glucose concentrations over the previous six to eight weeks, is likely to be raised. Occasionally, the diagnosis of GDM will identify women with type 1 diabetes mellitus6. Indeed, the incidence of type 1 DM is greater in pregnancy than in the background population7,8. One of the aims of the St Vincent Declaration is to ‘achieve pregnancy outcome in the diabetic woman that approximates to that of the non-diabetic woman’9. It is because of the adverse outcomes of diabetic pregnancy that screening is required.