Gestational diabetes is diagnosed through blood sugar testing, most commonly between 24 and 28 weeks of pregnancy. The process typically involves drinking a sugary glucose solution and having your blood drawn to measure how your body processes that sugar. Some women with higher risk factors may be tested earlier, but for most pregnancies, this window is when screening happens.
Why Testing Happens at 24 to 28 Weeks
The timing isn’t arbitrary. During pregnancy, the placenta produces hormones that help sustain the growing baby, but some of those same hormones, including estrogen, cortisol, and human placental lactogen, interfere with insulin’s ability to move sugar out of your blood and into your cells. This blocking effect kicks in around 20 to 24 weeks of pregnancy, which is why gestational diabetes rarely develops before that point.
Most women can compensate by producing extra insulin. Those who can’t keep up with the increased demand develop rising blood sugar levels, and that’s gestational diabetes. Testing at 24 to 28 weeks catches the condition right as it’s emerging, early enough to manage it effectively for the rest of the pregnancy.
Who Gets Tested Earlier
Some women are screened in the first trimester because they already carry risk factors for type 2 diabetes. These include obesity, a family history of type 2 diabetes, or having delivered an unusually large baby in a previous pregnancy. If your routine prenatal urine tests show elevated glucose, that can also prompt earlier screening. There’s no formal consensus on exactly when or how to screen before 24 weeks, so your provider will use clinical judgment based on your individual risk profile.
The Glucose Challenge Test (Step One)
In the United States, most providers use a two-step approach. The first step is a glucose challenge test, sometimes called a glucose screening. You drink a liquid containing 50 grams of glucose, which tastes like a very sweet flat soda. You don’t need to fast beforehand, which makes scheduling easier. One hour later, your blood is drawn to check your blood sugar level.
This first test is a screening, not a diagnosis. It sorts women into two groups: those whose blood sugar stayed within a normal range (and who are done with testing) and those whose levels were elevated enough to warrant a closer look. If your result comes back high, it doesn’t mean you have gestational diabetes. It means you need the second, more detailed test to find out.
The Oral Glucose Tolerance Test (Step Two)
The follow-up is a longer oral glucose tolerance test, or OGTT. This one requires preparation. For at least three days before the test, you should eat normally and include at least 150 grams of carbohydrates per day (roughly what most people eat anyway). Maintain your usual activity level and don’t exercise excessively. Then fast for at least eight hours before the test, drinking only water.
At the lab, your fasting blood sugar is drawn first. You then drink a glucose solution, this time containing either 75 or 100 grams depending on the protocol your provider follows. Blood is drawn again at one hour and two hours (and sometimes three hours with the 100-gram version). You can’t eat or smoke during the test, so expect to be at the lab for two to three hours total.
Your blood sugar is measured at each draw and compared against specific thresholds. Under the widely used international criteria for the 75-gram test, the cutoffs are:
- Fasting: 92 mg/dL or higher
- One hour: 180 mg/dL or higher
- Two hours: 153 mg/dL or higher
Meeting or exceeding just one of these values is enough for a gestational diabetes diagnosis under this approach. The 100-gram version used in the two-step method has its own set of thresholds, and typically requires two or more elevated values for a diagnosis.
One-Step vs. Two-Step Screening
Not all providers use the same method. The two-step approach (50-gram screening followed by a diagnostic OGTT if needed) is the most common in the U.S. The one-step approach skips the initial screening and goes straight to a 75-gram OGTT with the thresholds listed above. It catches more cases because its cutoffs are lower, but that also means more women receive a gestational diabetes diagnosis and the treatment that comes with it.
Both approaches are considered acceptable. Which one your provider uses depends on their practice guidelines and institutional preferences. You’ll likely have no say in which method is offered, but knowing the difference can help you understand what to expect and why your results look the way they do.
What Happens After the Test
If your results are normal, no further glucose testing is needed during pregnancy. If you’re diagnosed with gestational diabetes, your provider will work with you on a management plan that typically starts with dietary changes and blood sugar monitoring. Many women manage gestational diabetes through food choices and activity alone, though some need medication.
One thing many women don’t realize is that testing doesn’t end with delivery. A fasting blood sugar test alone can miss up to 40% of blood sugar problems after pregnancy, so the recommended follow-up is a full 75-gram oral glucose tolerance test between 6 weeks and 6 months postpartum. This checks whether your blood sugar has returned to normal or whether you’ve developed prediabetes or type 2 diabetes, which is more likely in women who had gestational diabetes. It’s one of the most skipped postpartum tests, but one of the most important for your long-term health.