The glucose test during pregnancy is typically done between 24 and 28 weeks of gestation. This is the standard window recommended by the American College of Obstetricians and Gynecologists (ACOG) for routine screening, though some people are tested earlier based on risk factors.
Why 24 to 28 Weeks
The timing isn’t arbitrary. During pregnancy, the placenta produces hormones, including estrogen, cortisol, and human placental lactogen, that interfere with how your body uses insulin. This interference typically kicks in around week 20 to 24 and intensifies as the placenta grows. By testing at 24 to 28 weeks, your provider catches the point where insulin resistance has developed enough to show up on a blood test but early enough to manage it if gestational diabetes is present.
Testing before 24 weeks in an average-risk pregnancy wouldn’t reliably detect gestational diabetes because the placental hormones haven’t ramped up enough yet. ACOG specifically recommends against routine early screening for this reason.
Who Gets Tested Earlier
If you have certain risk factors, your provider will likely screen you before 24 weeks, often at your very first prenatal visit. Risk factors that trigger early screening include:
- BMI in the overweight or obese range
- Previous gestational diabetes in an earlier pregnancy
- Family history of type 2 diabetes in a parent or sibling
- Age 35 or older
- Previous delivery of a baby weighing 9 pounds or more
- Polycystic ovarian syndrome (PCOS)
- Racial or ethnic background associated with higher type 2 diabetes risk, including American Indian/Alaska Native, Asian American, Black, Hispanic/Latino, and Native Hawaiian/Pacific Islander populations
This early screening looks for pre-existing (pregestational) diabetes that may not have been diagnosed before pregnancy. If your early test comes back normal, you’ll still be screened again at 24 to 28 weeks, since gestational diabetes can develop later as placental hormones increase.
What Happens During the 1-Hour Test
The initial screening is often called the glucose challenge test or the 1-hour test. You do not need to fast beforehand. At your appointment, you’ll drink a sweet glucose solution containing 50 grams of sugar. Most people describe the taste as flat orange soda or a very sweet fruit drink. One hour later, a blood sample is drawn to measure your blood sugar.
A result below 140 mg/dL is generally considered normal. A result of 190 mg/dL or higher indicates gestational diabetes without further testing needed. If your number falls somewhere between those two values, you’ll be asked to come back for the longer, more definitive test.
The 3-Hour Follow-Up Test
If your 1-hour screening comes back elevated but below 190, the next step is a glucose tolerance test, sometimes called the 3-hour test. This one requires fasting for at least eight hours beforehand, so most people schedule it first thing in the morning. Your fasting blood sugar is drawn first, then you drink a larger glucose solution (100 grams). 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 won’t be able to eat during the test, and most providers ask you to stay seated. Bring something to read or watch. Some people feel nauseous or lightheaded from the sugar on an empty stomach, which is uncomfortable but passes once the test is over and you eat something.
If two or more of the blood draws come back above the threshold values, gestational diabetes is diagnosed.
What to Know Before Your Appointment
For the 1-hour screening, eat normally in the days leading up to it. There’s no need to change your diet or avoid sugar beforehand. Some providers will let you choose a flavor for the glucose drink, so it’s worth asking. The test itself takes about an hour from the time you finish the drink, but factor in wait time at the lab.
For the 3-hour test, stop eating and drinking (except water, if your provider allows it) eight hours before your appointment. Many people eat a protein-heavy dinner the night before to avoid waking up extremely hungry. Avoid high-sugar meals the night before, as this can affect your fasting number.
About 2 to 10 percent of pregnancies involve gestational diabetes, so a positive screening result on the 1-hour test does not mean you have it. Many people who fail the 1-hour test pass the 3-hour test without issue. If you do receive a diagnosis, gestational diabetes is one of the most manageable pregnancy complications, typically controlled through dietary changes, blood sugar monitoring, and sometimes medication.