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The 75 gram 2 hour Oral Glucose Tolerance Test (OGTT)

At 24 to 28 weeks'

The two hour oral glucose tolerance test is performed after an overnight fast on all women not previously found to have overt diabetes or gestational diabetes (GDM) during testing earlier in their current pregnancy. To be applied to women without known diabetes before pregnancy.

The test should be done in the morning after an overnight fast of 8 - 14 hours and after at least 3 days of unrestricted diet (>150 grams carbohydrates per day) and unlimited physical activity.

  • If the mother's fasting blood sugar is very high some laboratories will cancel the test and report only the fasting blood sugar.

  • If the mother's fasting blood sugar is in an acceptable range , then she is given 75 grams of  glucose load (usually in the form of a drink).

  • Blood is collected at 1 and 2 hours after glucose load.

  • The mother should remain seated and should not smoke throughout the test.

Threshold values for diagnosis of GDM
Glucose concentration threshold*

Glucose measure mmol/l mg/dl

FPG 5.1 92
1-h plasma glucose 10.0 180
2-h plasma glucose 8.5 153

*One or more of these values from a 75-g OGTT must be equaled or exceeded for the diagnosis of GDM.

  • Diagnose as overt diabetes if fasting plasma glucose > 7.0 mmol/l (126 mg/dl)

  • Diagnose GDM if one or more values equals or exceeds the thresholds indicated above

  • Diagnose as normal if all values on the OGTT are less than the thresholds indicated above

Postpartum glucose testing should be performed for all women diagnosed with overt diabetes during pregnancy or GDM

Reprinted with permission of The American Diabetes Association.

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