To and fro, obstetricians and endocrinologists have long debated the relative value of diagnosing and treating gestational diabetes mellitus (GDM). No doubt, significant health advantages can follow from identifying and treating women who have GDM, including:
- protecting the fetus from macrosomia and a lifetime of excess body fat and obesity
- avoiding birth injury, such as shoulder dystocia, and life-long paralytic disability
- early recognition of a group of women at risk of type 2 diabetes mellitus, which can result in cardiovascular disease and premature death when undertreated.1-6
Setting thresholds is a key sticking point
A fundamental issue with establishing diagnostic criteria for GDM, however, is that a continuum relationship exists between, on one hand, the maternal circulating glucose concentration below a level diagnostic of type 2 diabetes mellitus and, on the other hand, such outcomes as macrosomia, neonatal hyperglycemia, preeclampsia, preterm delivery, shoulder dystocia, birth injury, hyperbilirubinemia, and admission to a neonatal intensive care nursery. That is why there’s been a need for an expert consensus panel to establish glucose cutoffs that separate a “normal” state from GDM, based on an analysis of benefits and risks.
In June 2008, the International Association of Diabetes and Pregnancy Study Group convened 225 experts, from 40 countries, to review data and establish new criteria for diagnosing GDM.7 The panel decided that its target for detailed analysis should be a maternal glucose concentration that resulted in an increased risk of 1.75 for various adverse outcomes.
PART 1: New criteria for making a diagnosis of GDM
Consequently, the Study Group consensus panel concluded that GDM should be diagnosed when any one of three tests is abnormal:
- fasting venous plasma glucose ≥92 mg/dL but <126 mg/dL
- 1-hour glucose after a 75-g oral glucose load (the oral glucose tolerance test [OGTT]) ≥180 mg/dL
- 2-hour glucose after a 75-g OGTT ≥153 mg/dL.
Note the implications of these conclusions on diagnosis: Among the findings of the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study,4 8.3% of subjects had a fasting venous plasma glucose ≥92 mg/dL, and would be diagnosed with GDM, and an additional 7.8% had a 1-hour or 2-hour glucose above threshold limits after an OGTT. In total, therefore, 11.1% of women in the HAPO study had one elevated result; 3.9% had two elevated results; and 1.1% had all three results elevated.
PART 2: New criteria for diagnosing overt diabetes in pregnancy
The Study Group consensus panel recommended using the following tests and thresholds to diagnose overt diabetes (not GDM) in pregnancy:
- fasting venous plasma glucose ≥126 mg/dL
- hemoglobin A1c ≥6.5%
- random plasma glucose ≥200 mg/dL.
If one of the tests listed above is abnormal, a confirmatory test is clinically appropriate.
Testing during first and second trimesters
The consensus panel recommends that, at the first prenatal visit, you measure the fasting venous plasma glucose, hemoglobin A1c, or random plasma glucose in either all women or high-risk women only. If the result indicates overt diabetes, provide treatment and follow-up as is standard for a pregnant woman who has pregestational diabetes mellitus. If the result is not diagnostic of overt diabetes and the fasting blood glucose level is ≥92 mg/dL but <126 mg/dL, then GDM should be diagnosed. If the fasting venous plasma glucose is <92 mg/dL, then you should perform a 75-g OGTT at 24 to 28 weeks’ gestation.
Interventions to prevent gestational diabetes
The percentage of pregnant women who have gestational diabetes mellitus (GDM) is increasing: In Massachusetts, from 1998 to 2006, the rate rose from 3.4% to 4.9%.
In part, we’ve seen this rise because minority women, older women, and overweight women—all of whom are at increased risk of GDM—account for a growing percentage of pregnant women.1 Given ongoing change in these birth demographics, the increase in the rate of GDM over the past decade will likely continue—even accelerate.
We can make a difference. Here is how.
Effective interventions for preventing GDM are to 1) optimize metabolic conditioning and body mass before pregnancy and 2) exercise and limit weight gain, consistent with fetal health, during pregnancy.2
In a large cohort study, a BMI >25 kg/m2 was associated with a relative risk of having a diagnosis of GDM of 2.25, compared with the risk in women whose BMI was <25 kg/m2.3
Abdominal obesity, as measured by waist-hip ratio, also appears to be independently associated with an increased risk of GDM. In a small clinical trial, exercise training during pregnancy—comprising 200 minutes of cycling a week at 65% of predicted aerobic capacity—reduced birth weight by 4%, improved maternal insulin sensitivity, and reduced concentrations of fetal cord insulin-like growth factors I and II.4
References
1. Anna V, van der Ploeg HP, Cheung NW, Huxley RR, Bauman AE. Sociodemographic correlates of the increasing trend in prevalence of gestational diabetes mellitus in a large population of women between 1995 and 2005. Diabetes Care. 2008;31(12):2288-2293.
2. Morisset AS, St-Yves A, Veillette J, et al. Prevention of gestational diabetes mellitus: a review of studies on weight management. Diabetes Metab Res Rev. 2010;26(1):17-25.
3. Yeung EH, Hu FB, Solomon CG, et al. Life-course weight characteristics and the risk of gestational diabetes. Diabetologia. 2010;53(4):668-678.
4. Hopkins SA, Baldi JC, Cutfield WS, McCowan L, Hofman PL. Exercise training in pregnancy reduces offspring size without changes in maternal insulin sensitivity [published online ahead of print March 24, 2010]. J Clin Endocrinol Metab. 2010;doi:10.1210/jc.2009-2255.