The American Diabetes Association recommends a 75g oral glucose tolerance test (simply known as an OGTT) with plasma glucose measured at 1 hour and 2 hours in the 24-28 weeks gestation window.

The OGTT should be performed in the morning after an overnight fast of at least 8 hours.

A 2-step approach is sometimes used by giving a 50 gm glucose challenge in a non-fasting state.  If the screening threshold is met or exceeded, the OGTT is performed.  Using the same procedure as the 1-step OGTT, but 100 gm glucose challenge is used and glucose levels are also tested at the 3 hour mark.  GDM is diagnosed when 2 or more glucose levels fall at or above the thresholds.

 

No food or drink 8-12 hours prior to test

Blood is drawn and tested for the level of glucose in blood

High glucose level=potential diabetes

 

Diagnosis of GDM

The diagnosis of gestational diabetes is based on the results of the OGTT using the parameters given below. This reflects the recommendations of both the International Association of Diabetes & Pregnancy Study Groups, and the American Diabetes Association based on the results of the Hyperglycemia Adverse Pregnancy Outcome (HAPO) Trial. However, the American Congress of Obstetricians and Gynecologists has not yet made a decision to support these guidelines.

 

The diagnosis is made when any of the following blood glucose values are exceeded:   Fasting > 95 mg/dl   1 hour after glucose load > 180 mg/dl   2 hours after glucose load > 155 mg/dl   3 hours > 140 mg/dl

 

The Hyperglycemia and Adverse Pregnancy Outcomes trial of 2008 examined 25,505 pregnant women from 15 centers worldwide in an effort to clarify the risk of adverse outcomes associated with glucose intolerance during pregnancy at levels less than overt diabetes. These women underwent a 75g oral glucose tolerance test and if their fasting blood glucose was 105 mg/dL or less, or their 2 hour value was < 200 mg/dL, they were included in the study. The figure below shows a strong continuous relationship between all glucose tolerance levels and the incidence of birth weights above the 90^th percentile; the actual percent of body fat above 90%, and cord serum C-peptide greater than 90 compared with the oral glucose tolerance test results.

 

Relationship of Fasting Glucose Test Value to Percentage of Adverse Outcomes