Gestational diabetes screening typically happens between 24 and 28 weeks of pregnancy. If you have certain risk factors, your provider may test you earlier, sometimes at your very first prenatal visit. The timing isn’t arbitrary: it lines up with a shift in how your body handles blood sugar during the second trimester.
Why Testing Happens at 24 to 28 Weeks
Starting around 20 to 24 weeks of pregnancy, the placenta ramps up production of hormones, including estrogen, cortisol, and human placental lactogen, that interfere with insulin’s ability to move sugar out of your blood and into your cells. As the placenta grows through the second and third trimesters, this insulin resistance increases. The 24-to-28-week window catches the point where blood sugar problems become detectable but early enough to manage them before they affect the baby.
Before this window, many women with gestational diabetes would test normal simply because the placental hormones haven’t built up enough to reveal the problem. Testing too late, on the other hand, leaves less time to control blood sugar and reduce complications.
Who Gets Tested Earlier
Some women are screened at their first prenatal appointment or during the first trimester. Your provider will likely recommend early testing if you:
- Had gestational diabetes in a previous pregnancy
- Previously delivered a baby weighing over 9 pounds
- Are overweight
- Have a family history of type 2 diabetes
- Have polycystic ovary syndrome (PCOS)
- Are African American, Hispanic or Latino, American Indian, Alaska Native, Native Hawaiian, or Pacific Islander
Early testing sometimes includes a blood test measuring your average blood sugar over the past two to three months (HbA1c). A level of 6.5% or higher suggests you may have had diabetes before becoming pregnant, not gestational diabetes that developed during pregnancy. This distinction matters because pre-existing diabetes requires different management from the start.
If early screening comes back normal, you’ll still be tested again at 24 to 28 weeks, since gestational diabetes can develop later as placental hormone levels climb. A high glucose reading in your urine during any routine prenatal visit can also prompt testing outside the standard window.
What the Tests Involve
Most providers in the United States use a two-step approach. The first step is a one-hour glucose challenge test. You drink a bottle of very sweet liquid containing 50 grams of glucose. You don’t need to fast beforehand, and you can eat normally that day. One hour later, your blood is drawn. If your blood sugar comes back between 130 and 140 mg/dL or higher (the exact cutoff varies by provider), you move on to the second step.
The second step is a three-hour glucose tolerance test. This one requires fasting for 8 hours beforehand, so most people schedule it first thing in the morning. Your blood is drawn once before you drink the glucose solution (this time containing 100 grams), then again at the one-hour, two-hour, and three-hour marks. You can’t eat during the test, though water is fine. A diagnosis of gestational diabetes requires at least two of the four blood draws to come back above specific thresholds: 95 mg/dL fasting, 180 mg/dL at one hour, 155 mg/dL at two hours, or 140 mg/dL at three hours.
Some providers use a one-step approach instead. This involves fasting and then drinking a 75-gram glucose solution, with blood drawn at fasting, one hour, and two hours. It’s shorter but still requires the same fasting preparation.
What the Test Feels Like
The glucose drink is intensely sweet, often compared to flat orange soda but much more concentrated. Nausea is the most common side effect, and some women experience dizziness or feel shaky afterward. These reactions are temporary and typically pass within a few hours. For the one-hour screening, expect to be at your provider’s office for about 90 minutes total. The three-hour test takes closer to four hours, so bring something to read or do while you wait between blood draws. You’ll be asked to stay seated and resting for the duration.
After Delivery: Follow-Up Testing
Gestational diabetes usually resolves once you deliver the placenta, but it significantly raises your risk of developing type 2 diabetes later. Because of this, a follow-up glucose tolerance test is recommended 4 to 12 weeks after delivery. This is a two-hour test using 75 grams of glucose, similar to the one-step screening done during pregnancy.
Research suggests that women with normal fasting blood sugar in the first 24 to 48 hours after giving birth have a very low chance (less than 1%) of being diagnosed with type 2 diabetes at their 6-to-12-week postpartum test. The risk of some degree of impaired glucose metabolism is higher, around 14%, which is why the follow-up test remains important even if your blood sugar looks good right after delivery.