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.
The GCT consists of measuring the serum glucose concentration 1 hour after the woman has drunk a 50 g (some use 75 g) oral glucose load. Values below 130 dg/mL are usually considered normal, values above 140 mg/dL are usually considered abnormal, and values between 130 and 140 mg/dL are "in between."
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):
Pregnant women with two or more elevated values are considered to have gestational diabetes mellitus (GDM), though even one elevated value increases certain risks.