Can I do a 1-hour glucose tolerance test (GTT) at 30 weeks of pregnancy?

From the Guidelines

Yes, you can do a 1-hour glucose tolerance test (GTT) at 30 weeks of pregnancy, as it is still a valuable screening tool for gestational diabetes at this stage, according to the most recent guidelines 1. This test is typically performed between 24-28 weeks of gestation, but it can still be beneficial at 30 weeks if you haven't had it earlier or if risk factors for gestational diabetes have emerged. The test involves drinking a sweet solution containing 50 grams of glucose, followed by a blood draw one hour later to measure your blood sugar response. No fasting is required beforehand, and you can schedule it at any time of day. If your result is elevated (typically above 130-140 mg/dL, though cutoffs vary by institution), your provider will recommend a follow-up 3-hour GTT to confirm a gestational diabetes diagnosis. Some key points to consider include:

• The American College of Obstetricians and Gynecologists (ACOG) recommends using a threshold of 130,135, or 140 mg/dL for the 1-h 50-g GLT 1.
• The diagnosis of gestational diabetes mellitus (GDM) can be made using either a one-step or two-step strategy, with the two-step strategy involving an initial 50-g GLT followed by a 100-g OGTT if the initial result is abnormal 1. Testing at 30 weeks still allows time for intervention if gestational diabetes is detected, which is important because uncontrolled high blood sugar during pregnancy can lead to complications such as excessive fetal growth, delivery complications, and increased risk of diabetes for both mother and child later in life. It's worth noting that different diagnostic criteria may identify different degrees of maternal hyperglycemia and maternal/fetal risk, and the optimal strategy for diagnosing GDM is still a topic of debate among experts 1. However, the most recent guidelines from the American Diabetes Association (ADA) recommend using the International Association of Diabetes and Pregnancy Study Groups (IADPSG) diagnostic criteria to optimize gestational outcomes 1.

From the Research

Glucose Testing During Pregnancy

• The 1-hour glucose tolerance test (GTT) is a common screening technique for gestational diabetes, typically performed between 24 and 28 weeks of pregnancy 2.
• However, there is evidence to suggest that early glucose screening, including the 1-hour GTT, may be beneficial in detecting gestational diabetes in certain high-risk populations, such as women over 30 years old or those with risk factors for diabetes 3.
• A study published in 1996 found that early gestational glucose screening may be beneficial in detecting gestational diabetes in patients over 30 years old who are black or who have risk factors for diabetes 3.
• Another study published in 1989 evaluated the reproducibility of the 1-hour 50 gm oral glucose screening test and found that it is moderately reproducible, but reliance should not be placed on a single normal test result, particularly among patients with risk factors 4.

Timing of Glucose Testing

• The American College of Obstetricians and Gynecologists (ACOG) recommends screening for gestational diabetes at 24-28 weeks of pregnancy, but some studies suggest that earlier screening may be beneficial in certain populations 5, 3.
• A study published in 2021 found that gestational diabetes had a prevalence of 11.8% in all trimesters of pregnancy in Pakistan, suggesting that earlier screening may be necessary in certain populations 5.
• However, there is limited evidence to support the use of the 1-hour GTT at 30 weeks of pregnancy, and more research is needed to determine the optimal timing of glucose testing during pregnancy.

Risk Factors and Glucose Testing

• Certain risk factors, such as maternal age over 30 years, black race, and presence of risk factors for diabetes, may increase the likelihood of gestational diabetes and necessitate earlier or more frequent glucose testing 2, 3.
• A study published in 1997 found that less than 1% of prenatal patients without risk factors for gestational diabetes were ultimately found to have gestational diabetes, suggesting that selective screening based on risk factors may be a reasonable approach 2.