The standard glucose test during pregnancy happens between 24 and 28 weeks of gestation. This is when most pregnant people are screened for gestational diabetes, regardless of whether they have symptoms. If you have certain risk factors, your provider may test you earlier, sometimes in the first trimester.
Why 24 to 28 Weeks
Gestational diabetes develops when your body can’t produce enough insulin to handle the increased demands of pregnancy. This insulin resistance naturally ramps up during the second trimester as the placenta grows and produces more hormones. Testing before 24 weeks in average-risk pregnancies hasn’t shown clear benefits. The U.S. Preventive Services Task Force recommends screening at 24 weeks or after, and most providers schedule the test sometime in the four-week window between weeks 24 and 28.
If you start prenatal care after 28 weeks, you’ll still be screened. The timing just shifts to whenever you begin receiving care.
Who Gets Tested Earlier
Some people are screened before 24 weeks because their risk of gestational diabetes is higher. Your provider may recommend early testing if you have a body mass index of 25 or above, had gestational diabetes in a previous pregnancy, have a family history of type 2 diabetes, or have polycystic ovary syndrome. Early screening can happen as soon as the first prenatal visit and sometimes involves a blood test called HbA1c, which measures your average blood sugar over the past two to three months. Research published in The Lancet Diabetes & Endocrinology found that an early HbA1c test can identify those at highest risk and allow earlier intervention, potentially reducing the need for the standard glucose drink test later on.
If early testing comes back normal, you’ll typically be retested at 24 to 28 weeks since the condition can still develop later in pregnancy.
What the Initial Screening Looks Like
The first test is called a glucose challenge test, and it’s straightforward. You drink a sweet liquid containing 50 grams of glucose, then have your blood drawn one hour later. You don’t need to fast beforehand, so you can eat normally that day. Most providers have you drink the solution in the office or lab and wait there for the hour.
The drink tastes like a very sweet, flat orange soda. Some people tolerate it fine, while others feel nauseated. If the sugar drink is difficult for you, some providers allow alternatives like a specific brand of cleaner glucose solution (sold as “The Fresh Test”), pure orange juice, glucose tablets, or even a measured amount of jelly beans. Ask your provider ahead of time if substitutes are an option at your practice.
Your blood sugar is measured from the one-hour blood draw. If your level comes back below the cutoff your provider uses (commonly around 130 to 140 mg/dL), you pass and no further testing is needed. Most people pass this initial screen.
What Happens If You Don’t Pass
A high result on the one-hour test doesn’t mean you have gestational diabetes. It means you need a longer, more detailed follow-up called a glucose tolerance test. This second test requires fasting overnight for at least eight hours. You’ll have your blood drawn first thing to get a fasting level, then drink a larger glucose solution (100 grams), and have blood drawn at one, two, and three hours after.
For a diagnosis of gestational diabetes on this three-hour test, at least two of the four blood draws need to meet or exceed these thresholds:
- Fasting: 95 mg/dL or higher
- 1 hour: 180 mg/dL or higher
- 2 hours: 155 mg/dL or higher
- 3 hours: 140 mg/dL or higher
If only one value is elevated, you typically won’t receive a gestational diabetes diagnosis, though your provider may recommend dietary changes or closer monitoring.
The One-Step Alternative
Some practices use a single, longer test instead of the two-step approach. This one-step method uses 75 grams of glucose, requires fasting, and involves blood draws at fasting, one hour, and two hours. Unlike the two-step version, only one elevated value is enough for a diagnosis: fasting above 92 mg/dL, one-hour above 180 mg/dL, or two-hour above 153 mg/dL. Your provider will let you know which approach they use. Both are considered valid screening methods.
What to Expect on Test Day
For the initial one-hour screening, plan to be at the lab or office for about 90 minutes total. Eat a normal meal a couple hours before if you’d like, but avoid loading up on sugary foods that morning, as this could skew your results. Some providers suggest eating protein-rich foods before the test to help settle your stomach.
For the three-hour follow-up, block out your entire morning. You’ll be fasting and sitting in the office for over three hours, so bring something to read or do. Some people feel lightheaded or nauseous during the longer test, which is normal given the fasting and the concentrated sugar. Eating a balanced meal the night before, with protein and complex carbohydrates, can help you feel more comfortable.
Results from either test usually come back within a day or two. If you’re diagnosed with gestational diabetes, management typically starts with dietary changes and blood sugar monitoring at home, and the majority of cases are controlled without medication.