Controversy grows over redefinition of gestational diabetes

To the Editor: Moynihan has recently suggested that gestational diabetes mellitus (GDM) is a “non-entity”1 and cited controversy regarding new diagnostic criteria, while McIntyre and Oats have defended the practice of screening for GDM.2 Robust data associate hyperglycaemia with adverse pregnancy outcomes;3 and randomised controlled trial data show a beneficial effect of screening and treatment on perinatal outcomes.4

However, it is short-sighted to consider only the immediate outcome of a pregnancy. GDM has very significant long-term consequences for mothers and their offspring. Half of all women diagnosed with GDM will later develop type 2 diabetes, and they also bear a substantially increased risk of future cardiovascular disease.5 Identification of GDM therefore provides an opportunity to intervene with a view to improving the long-term health of young women who are at risk. Furthermore, in-utero hyperglycaemia seems to cause fetal programming effects which increase the risk of type 2 diabetes in offspring — additive to the genetic transmission of diabetogenic traits from mother to child.

The newly proposed International Association of the Diabetes and Pregnancy Study Groups (IADSPG) diagnostic criteria for GDM continue to use the oral glucose tolerance test (GTT) but will identify a greater number of women with GDM based on an abnormal fasting, rather than post-load, blood glucose level (BGL). We recently undertook a retrospective review of the results of diagnostic GTTs performed in 10 801 pregnant women between 2008 and 2011 in Western Sydney (at Westmead Hospital, Nepean Hospital, Blacktown Hospital, Auburn Hospital, and Moaven and Partners Pathology [a private pathology service provider]). Based on the older Australasian Diabetes in Pregnancy Society (ADIPS) criteria, 15.7% of results were diagnostic of GDM (4.0% elevated fasting BGL, 13.9% elevated 2-hour BGL, 2.2% both). If the same results are reclassified using IADSPG criteria, 14.6% are diagnostic of GDM (8.6% elevated fasting BGL, 9.2% elevated 2-hour BGL, 3.2% both).

We also reviewed the obstetric outcomes of 541 women who attended Blacktown Hospital and had BGLs diagnostic of GDM according to either criteria. Women identified by IADSPG criteria had more macrosomic infants (17.0% v 5.4%), low birthweight infants (11.0% v 6.4%) and babies admitted to the special care nursery (45.5% v 23.2%) compared with women identified using ADIPS cut-offs (all P < 0.05 by the Fisher exact test). In other words, the IADSPG criteria seem to be better at identifying women who will have poor obstetric outcomes. The number diagnosed was no higher in this analysis (although this might change if 1-hour BGL cut-offs were to be included, as proposed by the IADSPG).

We argue that GDM remains an important clinical entity, and that it is worth getting the diagnosis right.