The glucose tolerance test for gestational diabetes is something of a second trimester rite of passage.
If your pregnancy has proceeded uneventfully so far, nearly all the standard tests are behind you. Only one standard test left. And for this final one (typically between 24-28 weeks), you get to choke down a disgustingly sweet drink* (think cough syrup without the bitter aftertaste) and have your blood drawn repeatedly. Sounds fun, right?
Two major methods of screening women for gestational diabetes exist: The two step method and the one step method. Interestingly, these are not equivalent. Which test you receives can have a big impact on whether or not you are diagnosed with gestational diabetes.
Most providers in the U.S. use the two-step method. This results in 7-9% of pregnant women in the U.S. being diagnosed with gestational diabetes each year.
If all these providers were to switch to the one-step method, the percentage of women diagnosed would double to 18% of all pregnant women according to the American College of Obstetricians and Gynecologists.
Does this represent a potential overdiagnosis or current underdiagnosis?
Before we dig into this question, let’s back up for a minute to define the tests.
What Causes Gestational Diabetes?
Why do we need to test all pregnant women for gestational diabetes? Well, for starters, blame the placenta—the organ that connects your baby to your blood supply—the biggest culprit behind the question “what causes gestational diabetes?”
Our bodies rely on a hormone called insulin to move glucose (sugar) from our blood into our cells, where it can be used for energy. Early in pregnancy, women begin producing more insulin.
Then, around 20 weeks of pregnancy, the placenta starts releasing hormones (cortisol, estrogen, and human placental lactogen) into your bloodstream. These hormones block the effects of insulin, making you insulin resistant. You need more and more insulin to move glucose (sugar) into your cells. To compensate, women’s bodies pump out extra insulin. In normal pregnancy, insulin needs can rise by 200-300%.
If your body cannot keep up with the rising need for insulin, your blood sugar rises, and you develop gestational diabetes.
Around 20 weeks of pregnancy, the placenta starts releasing hormones.
These hormones block the effects of insulin, making you insulin resistant.
When you are insulin resistant, you need more insulin to move glucose into your cells.
When you need more insulin to move glucose into your cells, your body pumps out even more insulin.
If your body can’t keep up with the rising need for insulin, your blood sugar rises, and you develop gestational diabetes.
The rise in insulin resistance starts around 20 weeks. That is why women are screened at 24-28 weeks of pregnancy.
But if you have a BMI over 25 and have other risk factors for gestational diabetes, your doctor may screen you for diabetes at your first prenatal visit. Even if you test negative then, you will be retested between 24 and 28 weeks.
Gestational Diabetes Screening Tests
The Two-Step Method
Step 1: The one-hour glucose tolerance test
- You do not need to do anything to prepare for this test.
- You will drink a sickeningly sweet (seriously disgusting) drink containing 50 grams of glucose (sugar).
- You will have your blood drawn after an hour.
- If your blood sugar is above the cutoff (typically 140 mg/dL) you will move on to step 2.
Step 2: The three-hour glucose tolerance test
- You will need to fast for 8-14 hours before this test (nothing to eat or drink except water).
- You will drink an even more sickeningly sweet (who’d have thought it was possible) drink containing 100 grams of glucose
- Your blood sugar will be tested before you drink the glucose drink, and at 1 hour, 2 hours, and three hours afterwards.
The One Step Method
The two-hour glucose tolerance test
- You will need to fast for 8-14 hours before this test (nothing to eat or drink except water).
- You will drink a drink containing 75 grams of glucose
- Your blood sugar will be tested before you drink the glucose drink, and at 1 hour and 2 hours afterwards
Interpreting Your Results
Two-Step Method
Results from Step 1: The one-hour test
- If you take the one-hour test and your blood glucose is less than the threshold (usually <140 mg/dl), you do not have gestational diabetes.
- If you fail the one-hour test (i.e., your blood sugars exceed the cutoff), you will go on to Step 2: the three-hour test.
- Failing the one-hour test does not mean you have gestational diabetes. Only 1 out 3 women who fail the first step will be diagnosed with gestational diabetes.
Results from Step 2: The three-hour test
If two or more of your blood values for three-hour test exceed the thresholds below, you will be diagnosed with gestational diabetes.
Thresholds for two-step method:
Step one:
- usually <140 mg/dl
Step two:
- Fasting: blood glucose is greater than 95 mg/dL (5.3 mmol/L)
- 1 hour: blood glucose is greater than 180 mg/dL (10.0 mmol/L)
- 2 hour: blood glucose is greater than 155 mg/dL (8.6 mmol/L)
- 3 hour: blood glucose is greater than 140 mg/dL (7.8 mmol/L)
NOTE: Although the two-step protocol suggests diagnosis when two or more values have to exceed the threshold, some doctors will diagnose you with gestational diabetes even if only one of your values at one of the time points exceeds the threshold. More often, doctors may counsel you to watch your diet and to increase your activity level. They may want to recheck your blood sugar later.
One-step method
If one or more of your values exceed the thresholds below, you will be diagnosed with gestational diabetes.
Thresholds for the one-step method:
- Fasting: blood glucose is greater than 92 mg/dL (5.1 mmol/L)
- 1 hour: blood glucose is greater than 180 mg/dL (10.0 mmol/L)
- 2 hour: blood glucose is greater than 153 mg/dL (8.5 mmol/L)
What’s the difference between these methods? And which should I prefer?
So, which method is better?
Answering that question requires us to go back to a point we made at the beginning: the percentage of women being diagnosed with gestational diabetes for each method. To refresh, two-step method = 7-9% of pregnant women being diagnosed; one-step method = 18% of women being diagnosed.
This is a balance between the likelihood and repercussions of overdiagnosis vs. underdiagnosis. Overdiagnosis can cause women who actually have a low risk of complications loads of unnecessary stress and anxiety. Underdiagnosis can lead to unmanaged gestational diabetes and put women and babies at risk.
Bearing those concerns in mind, the data favors the two-step method. The overdiagnosis from the one-step method does not seem to be catching the extra women who benefit from a diagnosis of gestational diabetes. The additional 9% women diagnosed with the one-step vs. the two-step method appear to face a low risk of complications commonly linked with gestational diabetes, like having an overly large baby or obstructed labor.
Since diagnosing gestational diabetes in those women may boost anxiety and stress without bringing any significant health benefits, overdiagnosing is likely more problematic than underdiagnosing.
