We also must improve our level of postdelivery care; a cost-effectiveness analysis of the IADPSG criteria demonstrated that the new one-step approach will not be cost-effective without it. This is not surprising. Moreover, it’s a logical imperative: In addition to preventing adverse outcomes in newborns, an important benefit of diagnosing GDM is the opportunity to prevent diabetes in the future.
Dr. Coustan is professor of obstetrics and gynecology at the Warren Alpert Medical School of Brown University in Providence, R.I.
COUNTERPOINT: Not yet.
Diagnosing GDM with a single-step approach – rather than a two-step approach – is a positive change. The problem lies not with the one-step 75-g OGTT, but largely with the new diagnostic threshold values for GDM proposed by the IADPSG as part of this new one-step approach.
Dr. Gerard H.A. Visser
The HAPO study – the international observational study on which the IADPSG based its recommendations – was a beautiful study. It provided valuable data on the relationship between OGTT values and adverse perinatal outcomes. Unfortunately, but not surprisingly, there was a continuous linear relationship among fasting, 1-hour, and 2-hour glucose values and the frequency of adverse outcomes.
With this linear relationship and no clear-cut off values for normal and abnormal test results, the threshold values are by definition arbitrary. The IADPSG chose to base its diagnostic thresholds on a 1.75-fold increase in the incidence of fetal macrosomia. Using these criteria, approximately 18% of pregnant women worldwide, and 17%-25% of pregnant women in the United States, would be diagnosed as having GDM.
At the current time, there is insufficient evidence that the benefits of such an enormous increase in GDM patients would outweigh the disadvantages.
Two randomized studies have shown that treatment of GDM improves outcomes – lowering the incidence of fetal macrosomia, mortality, birth trauma, and in one study, cesarean delivery – but we do not have strong evidence yet that diagnosing and treating minor glucose abnormalities in all women will significantly improve outcomes.
There also is a real risk of "overmedicalization." A recent article that details driving factors – technological changes detecting even smaller abnormalities, for instance, and conflicted panels writing expanded disease definitions (BMJ 2012;344:e3502) – is acutely applicable to GDM. With lower thresholds, we run the risk of overdiagnosis, which could result in increases in labor inductions and in elective and secondary cesarean deliveries – without proven benefit.
This is starting to happen in Belgium: Data that are as yet unpublished in the medical literature show that the number of labor inductions in women with GDM – women whose diagnoses were made using lower thresholds than previously employed – has been rising. The interventions are driven by concern about presumed large babies, but thus far, signs are lacking of adverse effects in the babies.
With respect to prevention of childhood obesity, we need to consider the fact that while GDM is related to obesity in the offspring, this appears to be mainly in the case of coexisting maternal obesity.
A study from Finland showed that overweight and abdominal obesity in adolescents was associated with maternal obesity, and even more so with the combination of obesity and GDM, but not with GDM itself (Diabetes Care 2010:33;1115-21). Similar U.S. data on childhood metabolic syndrome have been published (Pediatrics 2005:115;e290-6), and a recent meta-analysis has shown that the relationship between diabetes in pregnancy and childhood obesity disappeared after adjustment for maternal BMI (Diabetologia 2011;54:1957-66).
We know from HAPO data too that GDM and obesity are independent risk factors with synergistic effects on short-term outcomes such as preeclampsia, primary cesarean deliveries, macrosomia, increased cord C-peptide levels, and newborn body fat.
As my colleague Dr. Harold W. de Valk and I point out in a recent article, it might be best to use stricter diagnostic thresholds for obese women rather than for all women, given what we know thus far about these synergistic effects and the possibility of a larger effect of obesity on long-term development of offspring (Am. J. Obstet. Gynecol. 2013:208;260-4).
This principle is part and parcel of the so-called Ryan recommendations, which may provide a more sensible alternative to the IADPSG approach. Dr. Ryan proposes that we use a threshold based on a twofold increase in large for gestational age (for an overall incidence of GDM around 10% instead of 18%), that we consider evidence insufficient for the treatment of very mild increases of glucose, except for increases in obese women, and that we continue to collect data and study outcomes (Diabetologia 2011;54:480-6).
A randomized controlled trial stratified for maternal BMI, in which half of the women with OGTT results in between the two proposed diagnostic threshold values are treated, is mandatory.