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Gestational Diabetes Mellitus: Update on Screening

Gestational Diabetes Mellitus: Update on Screening

Abnormal glucose levels occur in about 3% to 10% of pregnant women. In approximately 90% of these women, the cause is gestational diabetes mellitus (GDM); in 8%, preexisting type 2 diabetes is responsible.

GDM is defined as glucose intolerance that either begins or is first recognized during pregnancy. This term is applied to glucose intolerance whether it began before the pregnancy or persisted afterward. As you can imagine, this definition isn’t the most clear.

1. When should we screen?

In pregnant women who are not known to have diabetes, the American Diabetes Association (ADA) recommends screening for GDM at 24 to 28 weeks of gestation.1 The US Preventive Services Task Force suggests screening at 24 weeks.2

2. How should we screen? Can we just use an HbA1c or plasma glucose level?

Shelve fasting plasma glucose, random glucose, and hemoglobin A1c (HbA1c)—none of these are recommended. Why? They are low-sensitivity tests for pregnant women.

The question of screening has been debated, and there are two strategies:

1. The one-step strategy

Obtain a 75-g oral glucose tolerance test (OGTT).

Check plasma glucose levels at fasting, 1, and 2 hours.

You can diagnose GDM if any of the following plasma glucose levels are at or above:

• Fasting: 92 mg/dL

• 1 hour: 180 mg/dL

• 2 hour: 153 mg/dL

2. The two-step strategy

Here is the two-step screening protocol:

1. Obtain a 50-g, 1-hour glucose challenge test; measure the plasma glucose level 1 hour after the test.

2. If this level is higher than 130 mg/dL, 135 mg/dL, or 140 mg/dL, request a 100-g, 3-hour OGTT. Using the threshold of 140 mg/dL yields a detection rate of about 80%; a cutoff of 130 mg/dL allows a rate of approximately 90%.

GDM is diagnosed when at least of two of the four glucose levels are met or exceeded (please see page S19 of the ADA Standards of Care, 2017).1

In 2013, the ADA recommended the one-step protocol. The American Congress of Obstetricians and Gynecologists supports the two-step test strategy. Evidence from studies bolsters both test approaches.

But as a National Institutes of Health consensus development conference noted in 2013, when it backed the two-step approach, there is a dearth of data from clinical trials showing benefits from the one-step protocol. The initial 50-g glucose test has the advantage of not requiring fasting. The ADA guidelines point out that longer-term outcome studies are in progress.

In my next post, I will delve into treatment options for GDM.


1. American Diabetes Association. Standards of Medical Care in Diabetes—2017. Diabetes Care. 2017;40(suppl 1):S19-S20.

2. United States Preventive Services Task Force. Gestational Diabetes Mellitus, Screening. Final Recommendation Statement. https://www.uspreventiveservicestaskforce.org/Page/Document/RecommendationStatementFinal/gestational-diabetes-mellitus-screening. Accessed May 8, 2017.

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