Testing for Gestational Diabetes

Shannon M. Clark, MD
February 25, 2022
Testing for Gestational Diabetes

If you are pregnant or are trying to conceive, one important test to be prepared for is the glucose tolerance test (GTT or glucola). This test screens for gestational diabetes mellitus (GDM), a condition in which carbohydrate intolerance develops during pregnancy.

How is GDM diagnosed?

The current standard of care in the U.S. calls for all pregnant persons to be screened for GDM with a laboratory-based screening test at 24–28 weeks of gestation. Early diagnosis and treatment of GDM can prevent life threatening complications for the pregnant person, fetus, and pregnancy. This is why testing is a standard practice in the U.S. You can refuse the GTT just as you can refuse anything in pregnancy, but be sure to know the potential consequences of undiagnosed GDM.

The GTT is a two-step test based on first screening with the administration of a 50-g oral glucose solution followed by a 1-hour venous glucose determination. Patients whose glucose levels meet or exceed an institution’s screening threshold then undergo a 100-g, 3-hour diagnostic GTT. GDM is most often diagnosed in patients who have two or more abnormal values on the 3-hour GTT.

One of the most important things to note is that pregnant persons without any risk factors can still get GDM simply because they have a placenta. Early pregnancy screening for undiagnosed type 2 diabetes, preferably at the initiation of prenatal care, is suggested in those patients who have risk factors for diabetes, including those with a prior history of GDM.

These risk factors include:

  • Physical inactivity
  • First-degree relative with diabetes
  • High-risk race or ethnicity (eg, African American, Latino, Native American, Asian American, Pacific Islander)
  • Have previously given birth to an infant weighing 4,000g (approximately 9 lb) or more
  • Previous gestational diabetes mellitus
  • Hypertension (140/90 mm Hg or on therapy for hypertension)
  • High-density lipoprotein cholesterol level less than 35 mg/dL (0.90 mmol/L), a triglyceride level greater than 250 mg/dL (2.82 mmol/L)
  • Women with polycystic ovarian syndrome
  • A1C greater than or equal to 5.7%, impaired glucose tolerance, or impaired fasting glucose on previous testing
  • Other clinical conditions associated with insulin resistance (eg, prepregnancy body mass index greater than 40 kg/m2, acanthosis nigricans)
  • History of cardiovascular disease

What are potential complications of GDM?

For pregnant persons who have GDM, pregnancy complications include preeclampsia, urinary tract infection, hydramnios, increased operative intervention and future diabetes mellitus. For the fetus, GDM is associated with macrosomia, congenital anomalies, metabolic abnormalities, and stillbirth.

What happens if you have GDM?

Just as with many things that can occur in pregnancy, if you have GDM, do not put pressure or guilt on yourself that you did something wrong. According to the Gestational Diabetes Mellitus, ACOG Practice Bulletin, Number 190, February 2018, it was “estimated that in 2009, 7% of pregnancies were complicated by any type of diabetes and that approximately 86% of these cases represented persons with GDM.” The key is to diagnose it in a timely manner so that you can receive the proper care to help control it for your and your fetus’s safety.

Is the glucose tolerance test beverage toxic & dangerous?

In recent years, non-medical social media influencers, bloggers, and other social media accounts have used various tactics to make people believe that the GTT is toxic or dangerous. This type of behavior is used to deepen the divide in the provider-patient relationship. It often benefits the person sharing the false information at the cost of the patient’s health. Avoid influencers, bloggers, and social media accounts who use these fear-based tactics.

One concern is that some glucola drinks contain brominated vegetable oil (BVO). The truth is that many glucola drinks do not contain BVO in the first place, and second, BVO is in many common foods like cake mixes, sports drinks, Jell-O, sauces, boxed macaroni and cheese, candy, chewing gum, butter, cereals, snack foods, cosmetics, beer and more. BVO in a GTT drink should not be a reason to avoid the screening and/or diagnostic GDM test. The GTT drink IS SAFE in pregnancy; however, undiagnosed gestational diabetes mellitus is not.

There are multiple types of the GTT drink. If you are still concerned about BVO or any other ingredient such as color dyes, talk to your obstetrical care provider. They will help you pick one that works for you.

Is GTT toxic?

Food dye in the GTT

Are there side effects of the test?

Some patients may experience nausea and/or vomiting. Serving the hyperosmolar glucola drink on ice may reduce nausea and vomiting. If you vomited during the GTT and are willing to come back another day for repeat testing, premedication with an antiemetic drug may allow the test to be completed.

Are there alternatives to the glucola drink?

Alternatives to the GTT should be reserved for those patients who cannot tolerate it. In these patients, home blood sugar monitoring may be the best alternative in those who are considered high risk for GDM. The best alternative for diagnosing GDM in low-risk patients who cannot tolerate the GCT is unknown. The proposed alternatives to GCT like jelly beans and other food alternatives, have not been studied in large populations. In addition, none of the alternatives have been validated or they perform poorly and therefore have not been endorsed by ADA or ACOG.

Let's wrap it up!

To summarize, the GTT is an important test and can be a life-saving screening for those who are found to have GDM. If you have questions or concerns about the specific drink you’ll be administered during the test, speak with your healthcare provider. Your healthcare provider can work with you to ensure that you are safe, comfortable, and fully educated on the test and process.

Shannon M. Clark, MD

Shannon M. Clark, MD

Shannon M. Clark, MD, MMS is a double board certified ObGyn and Maternal-Fetal Medicine Specialist, and founder of Babies After 35. In her roles as a clinician, educator and researcher at UTMB-Galveston, she focuses on the care of people with maternal and/or fetal complications of pregnancy. Dr. Clark has taken a special interest in pregnancy after the age of 35, which according to age alone, is considered a high-risk pregnancy.

Follow Shannon on TikTok @tiktokbabydoc, Facebook @babiesafter35, and Instagram @babiesafter35.

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