Answers in the Gestational Diabetes Diagnosis Controversy

By Eva Martin, MD of Elm Tree Medical, Inc.

Few pregnancy rituals are more bemoaned than the glucose tolerance test. You drink a nauseatingly sugary drink and then have a blood draw. No fun! At least the results are straightforward. Not so fast- how doctors read the results of this test vary from provider to provider. There are two different sets of criteria for diagnosing gestational diabetes. Does it matter which of the two criteria a doctor uses to determine whether you have gestational diabetes? New studies say yes- so which criterion for diagnosing is better?

There are two major sets of criteria for diagnosing gestational diabetes (GDM)- or diabetes that occurs during pregnancy due to differences in the way the body processes glucose during pregnancy. The first set of criteria is the National Diabetes Data Group (NDDG) criteria and the second is the Carpenter-Coustan criteria. The NDDG criteria are more stringent than the Carpenter-Coustan criteria. In other words, it’s easier to be diagnosed with gestational diabetes when using the Carpenter-Coustan criteria. About 30% to 50% more women will be diagnosed and thus subject to interventions and treatments using the Carpenter-Coustan criteria than NDDG. Is treating more women at lower thresholds a good thing? Do these women in the borderzone have better pregnancy outcomes if they are treated or left alone?

Prior studies have shown that women who are diagnosed by the more inclusive Carpenter-Coustan criteria have higher risk of forceps or vacuum delivery, cesarean section, large babies, the shoulder getting stuck at delivery, and high blood pressure in pregnancy. Therefore, we know these women are at higher risk of pregnancy complications, but we do not know if providing them with treatment for gestational diabetes actually lowers these risks. Until now.

Dr. Harper and colleagues at the National Institutes of Health Maternal-Fetal Medicine collaborative published the results of a large study on the differences if you treat women diagnosed by these two major criteria. The study included 958 women. 59% were diagnosed with GDM by NDDG criteria (and by default, also Carpenter-Coustan criteria). 42% were diagnosed with GDM by Carpenter-Coustan but did not meet the thresholds by NDDG. The authors divided these two groups into women who received usual care and those who received treatment for GDM. This way, they could compare the effects of providing GDM treatments to women who met only the less stringent Carpenter-Coustan criteria but not NDDG. If a physician uses NDDG criteria, then these women would not usually receive treatment.

The authors of the study discovered that all of the women benefitted from treatment. The risks of complications were decreased similarly for women diagnosed by both criteria and those diagnosed by only the Carpenter-Coustan criteria. Only small numbers of women needed to be treated in both groups to prevent complications. For instance, treating 7 women who met only Carpenter-Coustan criteria prevents one cesarean section, and treating 16 women meeting only Carpenter-Coustan criteria prevents one case of macrosomia (or a large baby).

The results of the study are important for the everyday care of pregnant women and suggest that doctors should consider using the more stringent Carpenter-Coustan criteria for diagnosing gestational diabetes since women diagnosed by only this criteria also benefited from treatment.