Before and during pregnancy there is usually a two-step approach to diagnosing diabetes: The initial screening test is called GCT, or Glucose Challenge Test, and it's often done between 24 and 28 weeks of pregnancy.
The GCT does not necessarily diagnose diabetes but it screens women who may be at risk and who may need a 3-hour glucose tolerance test or GTT.
A glucose threshold value >140 mg/dL identifies approximately 80% of women with pregnancy diabetes (GDM), and the yield is further increased to 90% by using a cutoff of >130 mg/dL. Women whose serum glucose is above the threshold value on the GCT (130-140 mg/dL) have a diagnostic 3-hour GTT done to find out if they have diabetes. The 3-hour GTT consists of a woman drinking 100 g of glucose and having four blood samples drawn: fasting, and then 1, 2, and 3 hours after the drink. Cutoff values for the 3-hour GTT are as follows (Sullivan criteria):
- Fasting: 95 mg/dL
- 1-hour: 180 mg/dL
- 2-hours: 155 mg/dL
- 3-hours: 140 mg/dL
Pregnant women with two or more elevated values are considered to have gestational diabetes mellitus (GDM), though even one elevated value increases certain risks.