What is the role of oral glucose tolerance testing (GTT) in the diagnosis of gestational diabetes?

Updated: May 10, 2021
  • Author: Jiun-Lih Jerry Lin, MBBS, MS(Orth); Chief Editor: Eric B Staros, MD  more...
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Oral GTTs are commonly used to screen women for gestational diabetes. There is a strong correlation between increasing maternal glucose levels at 24–32 weeks’ gestation and a range of adverse maternal and fetal outcomes. [11] Several different strategies are used worldwide in the screening and diagnosis of gestational diabetes. 

The American Diabetes Association offers two approaches to gestational diabetes screening; a "one-step" approach with a single 2 hour 75 g OGTT, or a "two-step" approach with a 1 hour 50 g screening OGTT followed by a confirmatory 3 hour 100 g OGTT in patients who screen positive. [4]

The Australian Diabetes in Pregnancy Society (ADIPS) Consensus Guidelines for the Testing and Diagnosis of Gestational Diabetes Mellitus in Australia [12] recommend early testing for gestational diabetes mellitus in women with risk factors, in a tiered approach to early glucose testing.

Moderate risk factors include the following:

  • Ethnicity: Asian, Indian subcontinent, Aboriginal, Torres Strait Islander, Pacific Islander, Maori, Middle Eastern, non‐white African

  • Body mass index (BMI) of 25–35 kg/m2

High risk factors include the following:

  • Previous gestational diabetes mellitus

  • Previous elevated blood glucose value

  • Maternal age of 40 years or older

  • Family history diabetes mellitus (first-degree relative with diabetes mellitus or a sister with gestational diabetes mellitus)

  • BMI higher than 35 kg/m2

  • Previous macrosomia (baby with birth weight >4500 g or >90th percentile)

  • Polycystic ovarian syndrome

  • Medications (corticosteroids, antipsychotics)

Women with one moderate risk factor should initially be screened with a random or a fasting glucose test in early pregnancy, followed by pregnancy oral GTT if clinically indicated. High-risk (one or more high risk factor or 2 moderate risk factors) women should undergo 75 g pregnancy oral GTT, with venous plasma samples taken fasting, 1 hour and 2 hours, and at the first opportunity after conception.

Moderate- or high-risk women with normal early pregnancy glucose testing should have repeat pregnancy oral GTT at 24–28 weeks’ gestation [13] or earlier if indicated. As risk stratification by risk factors is unreliable, all women should be tested, and all women not known to have gestational diabetes mellitus should have 75 g pregnancy oral GTT at 24–28 weeks.

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