Most pregnant women have their glucose screening test between 24 and 28 weeks of pregnancy. This is the standard window recommended by major medical organizations, though your provider may test you earlier if you have certain risk factors. Here’s what to expect and how the process works.
Why 24 to 28 Weeks
The placenta produces hormones that can interfere with how your body uses insulin, and this effect ramps up during the second half of pregnancy. By 24 weeks, the hormonal shift is significant enough that blood sugar problems will show up on a test, but early enough that treatment can make a real difference for you and your baby. Testing before this point in a low-risk pregnancy would miss many cases that develop later.
Who Gets Tested Earlier
If you’re at higher risk for gestational diabetes, your provider may screen you at your first prenatal visit or before 15 weeks. The CDC lists these risk factors:
- Previous gestational diabetes in an earlier pregnancy
- Previous baby weighing over 9 pounds at birth
- Being overweight before pregnancy
- Family history of type 2 diabetes
- PCOS (polycystic ovary syndrome)
- Certain racial and ethnic backgrounds, including African American, Hispanic or Latino, American Indian, Alaska Native, Native Hawaiian, or Pacific Islander
Your provider might also move the test up if routine urine checks at prenatal visits show elevated glucose levels. If early testing comes back normal, you’ll typically be tested again at 24 to 28 weeks.
The One-Hour Screening Test
The most common approach in the U.S. is a two-step process. Step one is a quick screening: you drink a glucose solution containing 50 grams of sugar (about the sweetness of a flat orange soda), then have your blood drawn one hour later. You don’t need to fast beforehand, and you don’t need to schedule it at any particular time of day. That convenience is one reason this method is so widely used.
A blood sugar reading below 140 mg/dL is considered normal. Some clinics use a slightly lower cutoff of 130 mg/dL. A result of 190 mg/dL or higher points to gestational diabetes without further testing. Anything between those numbers means you’ll need a longer follow-up test to get a definitive answer. Roughly 15% to 20% of women who take the one-hour screening end up needing that second test, so if it happens to you, it doesn’t mean you have gestational diabetes.
The Three-Hour Diagnostic Test
If your one-hour result is elevated but below 190 mg/dL, the next step is a three-hour glucose tolerance test. This one does require fasting overnight, typically for at least eight hours. At the appointment, your fasting blood sugar is drawn first. Then you drink a more concentrated glucose solution containing 100 grams of sugar. Your blood is drawn again at one hour, two hours, and three hours after you finish the drink.
Plan to be at the lab or clinic for the full three hours. You’ll need to stay seated and can’t eat during the test. Bring something to read or watch, because it’s a long sit.
The One-Step Alternative
Some practices skip the initial screening and go straight to a single two-hour test. You fast overnight, drink a 75-gram glucose solution, and have blood drawn at one hour and two hours. This approach requires only one visit instead of potentially two, but it does require fasting. It also identifies gestational diabetes at about two to three times the rate of the two-step method, meaning more women end up diagnosed and treated. Your provider will decide which approach to use based on their practice guidelines.
How the Drink Feels
The glucose solution is intensely sweet, and up to 30% of women experience side effects like nausea. Drinking it quickly can make the nausea worse, though it usually passes within a few minutes. Some women feel faint or dizzy, particularly during the fasting three-hour test, simply because they haven’t eaten. If that happens, let the staff know right away.
Some providers allow alternatives to the standard glucose drink. Options that certain clinics accept include a cleaner-ingredient glucose solution (sold under the brand Fresh Test), jelly beans with a measured sugar content, or pure orange juice. These are not universally approved, so ask your provider beforehand if you’re concerned about tolerating the standard drink. The key is consuming the right amount of sugar in a controlled time frame so the results are reliable.
What Happens After Your Results
If your results are normal, no further glucose testing is needed until your next pregnancy. If you’re diagnosed with gestational diabetes, the goal is to keep your blood sugar within specific targets: fasting levels under 95 mg/dL and post-meal levels under 140 mg/dL at one hour or under 120 mg/dL at two hours. Most women manage this with dietary changes and regular blood sugar monitoring. Some need insulin if food adjustments alone aren’t enough.
Women diagnosed with gestational diabetes are also screened again at 4 to 12 weeks after delivery, using a two-hour glucose tolerance test, to make sure blood sugar levels have returned to normal. A history of gestational diabetes raises your long-term risk of developing type 2 diabetes, so ongoing screening every one to three years is generally recommended after that.