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Screening and Diagnosis of Gestational Diabetes Mellitus

– Posted in: General Pregnancy Topics, Nutrition/Diet
Gestational Diabetes Mellitus Testing Blood Sugar

All of the screening and diagnostic tests for diabetes involve drinking glucose-containing drinks and blood glucose measurements. If the tests identify gestational diabetes, you will be informed and asked to change your diet, requested to make more frequency prenatal visits, start monitoring your blood glucose levels at home, possible start insulin therapy, and have additional maternal and fetal monitoring.

Who should be screened for gestational diabetes?

The American College of Obstetrics and Gynecologists recommends that every pregnant women should be screened for gestational diabetes.

During pregnancy when should diabetes screening be performed?

The American College of Obstetrics and Gynecologists recommends in the absence of a high degree of suspicion for undiagnosed diabetes that universal screening be performed at 24 to 28 weeks of pregnancy.  You can be screened as early as your first prenatal visit if you are obese, history of previous gestational diabetes, sugar in your urine sample, or a family history of diabetes. Specifically, if you have a prior pregnancy complicated by diabetes you are tested early due to a 33 to 50 percent chance of recurrence.

You will also be screened early at your first prenatal visit if any of the following are found:

  • Fasting glucose greater than126 mg/dL
  • A specific lab test for diabetes called A1C is greater than 6.5 percent
  • A Random not fasting blood glucose of greater than 200 mg/dL  

How to screen for gestational diabetes

There is not an accepted worldwide standard for diabetic screening in pregnancy.  The current approach in the United States is to first do a screening test and followed it by a diagnostic test if the screen was positive. (called a two-step approach).

Two step approach

The two step approach starts with a 50 gram glucose drink that is given without concern for the time of your last meal.  Your glucose blood level is measured one hour later. This is called a “one-hour GTT”.  If your glucose level is less than 130 mg/dl then you do not have gestational diabetes and no further testing is required.  On the other hand if your glucose level is greater than 130 mg/dl then you will undergo further testing called glucose tolerance test. This diagnostic test is definitive for the diagnosis of gestational diabetes.


The oral GTT is a practical means of diagnosing gestational diabetes. It can be administered by two different methods.

100 gram three hour oral glucose tolerance test

The 100 gram three hour oral GTT is most commonly used during pregnancy in the United States. The test is recommended by both ACOG and by a 2013 NIH Consensus Conference. For the test to show you have diabetes you need to have two elevated glucose values.

The diagnostic criteria for the 100 gram 3-hour GTT to diagnose gestational diabetes mellitus:

  • Fasting blood sugar greater than 95 mg/dl
  • One hour blood sugar greater than 180 mg/dl
  • Two hour blood sugar greater than 155 mg/dl
  • Three hour blood sugar greater than 140 mg/dl

75 gram two hour oral glucose tolerance test

The 75 gram two hour oral GTT is recommended by the IADPSG and ADA  (International Association of Diabetes and Pregnancy Study Groups (IADPSG) and American Diabetes Association). The reduce glucose in this test is more convenient, better tolerated, and more sensitive for identifying the pregnancy at risk for diabetes than the 100 gram three hour oral GTT. The Increased predictability of this test  is likely related to a lower threshold for a positive test – only one elevated glucose value is needed and the cut-offs for a positive result are slightly lower.

Criteria for a positive two hour 75 gram oral glucose tolerance test:

  • Fasting blood sugar greater than 92 mg/dl
  • One hour blood sugar great than 180  mg/dl
  • Two hour blood sugar greater than 153 mg/dl 


The glucose drink is very concentrated and not well tolerated.  The drink can cause nausea and even vomiting that make the glucose screening unreliable.  Alternative to the glucose drink such as candy, a calculated calorie meal, or soft drink are better tolerated, but less reliable as an indicator of diabetes. None of the alternative glucose loads have been endorsed by the ADA or ACOG.

1 Comment… add one

October 4, 2013, 6:19 am Jeet Chowhan

Schedule regular visits with your healthcare provider. If you have been diagnosed with gestational diabetes mellitus, you may need additional prenatal care visits throughout your pregnancy so that your physician or healthcare team can teach you how to take care of your new condition.f you have never had gestational diabetes before, following a new dietary, exercise, and medical regimen may be confusing and difficult. Having gestational diabetes means you must pay special attention to everything you eat, and learn how to manage your diet so that your blood sugar stays within a healthy range.Monitor your blood glucose level carefully.Eat a balanced diet and avoid simple carbohydrates.One of the most effective ways to control blood glucose levels is to engage in regular physical activity. If you have little experience exercising or if you are uncomfortable doing significant activity, try starting with regular walks.