Gestational diabetes testing involves specific blood sugar cutoffs, and whether your results are normal or abnormal depends on which test you took. Most providers in the U.S. use a two-step process: a quick screening test first, followed by a longer diagnostic test only if the screening flags a concern. Some practices use a single, longer test instead. Here’s how to read the numbers from each one.
The Two Testing Methods
The most common approach is the two-step method. Step one is a quick screening where you drink a sugary solution containing 50 grams of glucose, then have your blood drawn one hour later. You don’t need to fast beforehand. If your blood sugar comes back above the cutoff, you move on to step two: a longer, fasting test that measures your blood sugar four times over three hours.
The less common approach is a one-step test. You fast overnight, drink a 75-gram glucose solution, and have your blood drawn at fasting, one hour, and two hours. This method is used by some providers who follow international guidelines. The cutoffs are different from the two-step method, so knowing which test you took is essential for reading your results correctly.
Step One: The 1-Hour Screening Test
This initial screening uses a single blood draw taken one hour after you drink the glucose solution. Most providers set the cutoff at 140 mg/dL. If your result is below 140, you screen negative and no further testing is needed. About 70% of pregnant women fall into this category.
Some providers use a lower cutoff of 130 mg/dL, particularly for women considered higher risk. If your result falls between 130 and 139, whether you need the follow-up test depends on which threshold your provider uses. Ask your office which cutoff they apply.
A result above the cutoff does not mean you have gestational diabetes. It means your blood sugar was high enough to warrant the longer diagnostic test. Many women who fail the screening go on to pass the three-hour test.
Step Two: The 3-Hour Diagnostic Test
If your screening was elevated, you’ll be scheduled for a 100-gram oral glucose tolerance test. This one requires fasting for at least eight hours beforehand. Your blood is drawn four times: once fasting (before you drink the solution), then at one hour, two hours, and three hours after.
The standard cutoffs, known as the Carpenter-Coustan criteria, are:
- Fasting: 95 mg/dL or higher is abnormal
- 1 hour: 180 mg/dL or higher is abnormal
- 2 hours: 155 mg/dL or higher is abnormal
- 3 hours: 140 mg/dL or higher is abnormal
To receive a gestational diabetes diagnosis, two or more of these four values must meet or exceed the cutoffs. If only one value is elevated, you technically don’t meet the diagnostic threshold, though your provider may still recommend dietary changes or closer monitoring depending on how high that single value was.
If all four values are below the cutoffs, you do not have gestational diabetes. Your results are normal.
The One-Step 75-Gram Test
If your provider uses the one-step method recommended by the International Association of Diabetes and Pregnancy Study Groups, you’ll fast overnight and then drink a solution with 75 grams of glucose. Blood is drawn three times: fasting, at one hour, and at two hours.
The cutoffs for this test are:
- Fasting: 92 mg/dL or higher
- 1 hour: 180 mg/dL or higher
- 2 hours: 153 mg/dL or higher
The key difference here is that only one abnormal value is enough for a diagnosis. This makes the one-step test more sensitive, meaning it catches more cases. If you’re comparing notes with a friend who took a different version of the test, this is why your experiences might not line up.
When Testing Happens
Routine screening typically takes place between 24 and 28 weeks of pregnancy. This is when pregnancy hormones from the placenta are affecting insulin function enough for the test to be meaningful.
If you have risk factors like a previous gestational diabetes diagnosis, a family history of type 2 diabetes, or a high BMI, your provider may test earlier, sometimes in the first trimester. Early testing before 15 weeks of gestation may use a standard glucose tolerance test or a hemoglobin A1c blood test. An A1c of 6.5% or higher suggests diabetes that likely existed before pregnancy rather than gestational diabetes that developed during it. Some research has identified 5.7% as a more sensitive early cutoff, though it also produces more false positives.
What Can Affect Your Results
For the screening test, you eat normally beforehand, so what you had for breakfast shouldn’t dramatically skew your numbers in most cases. For the fasting tests (the three-hour or the 75-gram), you need to go without food or drink (except water) for at least eight hours. Eating during the fasting window will invalidate your results.
In the days before either test, eat your normal diet. Some women try to restrict carbs beforehand thinking it will help them “pass,” but this can actually backfire. If your body hasn’t been processing normal amounts of carbohydrates, it may overreact to the sudden glucose load and produce a falsely high reading. Illness and certain medications can also shift your numbers, so let your provider know if either applies.
What Your Numbers Mean Day to Day
If you are diagnosed with gestational diabetes, the diagnostic test cutoffs are not the same as the daily blood sugar targets you’ll be asked to maintain. Home monitoring uses tighter ranges to keep both you and the baby healthy throughout the rest of the pregnancy.
Typical daily targets for gestational diabetes are a fasting blood sugar below 95 mg/dL (measured first thing in the morning before eating) and a reading below 140 mg/dL one hour after starting a meal, or below 120 mg/dL two hours after starting a meal. Your provider will tell you which post-meal timing to use. These targets are stricter than what’s recommended for type 2 diabetes outside of pregnancy, because blood sugar crosses the placenta and directly affects the baby’s growth.
Most women manage gestational diabetes with dietary changes and regular activity. You’ll typically be asked to check your blood sugar several times a day, including fasting and after each main meal. If your numbers consistently run above target despite dietary adjustments, your provider may add insulin or another medication. The condition resolves after delivery in the vast majority of cases, though it does raise your long-term risk of developing type 2 diabetes later in life.