Gestational diabetes mellitus (GDM)

1 Defining the condition
The widely accepted definition is that given by the American Diabetes Association (ADA) ‘…any degree of glucose intolerance with onset or first recognition during pregnancy’ [5]. The definition is applicable even if ‘the condition persists after pregnancy’. ‘It does not exclude the possibility that unrecognized glucose intolerance may have antedated or begun concomitantly with the pregnancy.’ The widespread acceptance of this definition is in no small part due to the fact that it does not mention any specific diagnostic criteria. Any definition of GDM has to take into account three elements of risk – of perinatal morbidity and mortality in the current pregnancy, of the mother developing type 2 diabetes, and of intra-uterine programming of the developing fetus with subsequent expression of disorders in later life.

2 Diagnosis of GDM
There is a continuum of risk for maternal glucose levels and, at least, adverse pregnancy outcomes [6-11]. Currently there is a lack of international consensus regarding the diagnostic
criteria for GDM. In most parts of the world the diagnostic criteria are based on either the 100 gram 3-hour test as commonly used in the USA or the 75 gram 2-hour World Health Organization (WHO) test. Many national bodies have derived their own criteria based on local experience and their healthcare delivery systems. This lack of consensus may well be addressed by recommendations arising from the International Association of Diabetes in Pregnancy Study Groups (IADPSG), a working group analysing the results of the HAPO study. Any recommendations from this group will then need to be considered by relevant national
bodies and incorporated into the local health service arrangements. This process will take some time. Other than by chance, it is not clear if any diagnostic criteria of GDM
based exclusively on pregnancy outcomes will be applicable to the other two elements of risk.

3 Rationale for treating GDM
It is generally acknowledged that women with GDM are at increased risk of adverse pregnancy outcomes, particularly relating to perinatal mortality and morbidity. It
is also generally acknowledged that treatment of women with GDM, by whatever means, can reduce the risk of these problems. In the developed world an increased perinatal mortality rate is unlikely but can still be demonstrated in a sufficiently large series [12]. However, in
settings where obstetric care does not uniformly reach modern quality standards, perinatal mortality is still an important issue [13].
Perinatal morbidity is an ongoing concern. Macrosomic or large-for-gestational-age (LGA) infants are still common, and can be considered a surrogate marker for at least
some of the effects of intra-uterine programming.  An earlier prospective controlled trial demonstrated that ‘tight’ control, with a high rate of insulin use, improved perinatal
outcomes [14]. Later, a prospective non-randomized intervention study demonstrated for women with GDM that intensive control (versus conventional control) improved
perinatal outcomes to a level that was comparable to a group without GDM [15]. The hazards of a late diagnosis of GDM, and therefore effectively no treatment, have been
outlined [16]. The Australian Carbohydrate Intolerance Study in Pregnancy (ACHOIS), a blinded randomized trial including 1000 women, designed to examine whether the treatment
of women with GDM would reduce perinatal complications, found a significant reduction in serious perinatal complications in the treated group [17]. Recently the results of the
Maternal-Fetal Medicine Unit (MFMU) Network study have become available. Treating women with designated ‘mild’ GDM lowered the risk for many adverse pregnancy outcomes [18].
Limited observational studies in humans strongly suggest that any pregnancy complicated by hyperglycaemia confers a risk to the offspring of developing type 2 diabetes [19-24],
and that improving maternal glycaemic control may reduce this risk. However, the long follow-up necessary makes it unlikely that any randomized controlled trial (RCT) evidence will be forthcoming in the foreseeable future