The glucose test during pregnancy is done between 24 and 28 weeks of gestation. This timing applies to most pregnant people, though those with certain risk factors may be screened earlier, sometimes as soon as their first prenatal visit.
Why 24 to 28 Weeks
Gestational diabetes usually develops around the 24th week of pregnancy. During pregnancy, your body produces more hormones and undergoes changes like weight gain that make your cells respond less effectively to insulin. This insulin resistance increases as pregnancy progresses, and all pregnant women experience some degree of it during late pregnancy. By 24 weeks, the resistance is significant enough that screening can reliably detect whether your body is keeping up with the increased demand for insulin.
Testing earlier than 24 weeks in an average-risk pregnancy wouldn’t catch the problem, since the hormonal shifts driving insulin resistance haven’t fully kicked in yet. Testing much later leaves less time to manage blood sugar if gestational diabetes is found.
Who Gets Tested Earlier
If you’re at higher risk for gestational diabetes, your provider may screen you before 24 weeks, potentially at your very first prenatal appointment. Risk factors that can trigger early screening include being overweight or obese before pregnancy, having a parent or sibling with diabetes, or having had gestational diabetes in a previous pregnancy.
If you have a history of gestational diabetes and are planning another pregnancy, guidelines from the American Diabetes Association recommend screening for type 2 diabetes or prediabetes before conception. If that preconception screening doesn’t happen, testing should be done before 15 weeks of gestation. Research is still ongoing about whether treating mildly elevated blood sugar found this early in pregnancy improves outcomes, so your provider may recommend nutrition counseling and periodic blood sugar monitoring rather than immediate intensive treatment.
What Happens During the One-Hour Test
The initial screening is called the glucose challenge test. You drink a liquid containing 50 grams of glucose (it tastes like a very sweet flat soda), and then your blood is drawn one hour later to measure your blood sugar. You don’t need to fast beforehand, which is one reason this test works well as a routine screen. You can eat normally before your appointment.
The cutoff for a normal result varies slightly by practice. Screening thresholds range from 130 to 140 mg/dL, and there’s no single universally agreed-upon number. Your provider’s office will use one consistent cutoff, typically either 135 or 140 mg/dL. If your blood sugar comes back below that threshold, you’re done. No further testing is needed.
If Your One-Hour Result Is High
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 the oral glucose tolerance test. For this second test, you do need to fast overnight. You’ll drink a stronger glucose solution (100 grams), and your blood is drawn at multiple intervals: fasting, then at one hour, two hours, and three hours after drinking the solution.
A gestational diabetes diagnosis requires two or more of those four blood draws to come back above the threshold values. If only one is elevated, the result is considered normal. Many women who fail the initial one-hour screening pass the three-hour test without any issues.
What Happens if You Start Prenatal Care Late
If you begin prenatal care after 28 weeks, the glucose screening will still be done. The U.S. Preventive Services Task Force recommends screening at 24 weeks or after, so entering care late doesn’t disqualify you from testing. Your provider will likely schedule the screening promptly, since later detection still allows time for dietary changes, blood sugar monitoring, and treatment if needed to reduce risks for the remainder of pregnancy.