A pregnancy glucose test is a routine blood test that checks for gestational diabetes, a type of high blood sugar that develops during pregnancy. Most pregnant women take this test between 24 and 28 weeks of pregnancy, though it may be done earlier if you have risk factors like a high glucose level in your urine at a prenatal visit or a family history of diabetes.
Why the Test Matters
Gestational diabetes often has no obvious symptoms, which is why screening is standard for nearly all pregnancies. High blood sugar during pregnancy increases the risk of the baby growing too large, which can complicate delivery and raise the likelihood of a cesarean birth. It also increases the baby’s risk of developing obesity or type 2 diabetes later in life. For the mother, uncontrolled blood sugar raises the chance of preterm birth and other complications.
The good news is that gestational diabetes is very manageable once it’s caught. Most women control it through dietary changes and monitoring, and blood sugar typically returns to normal after delivery. The test exists to catch the problem early enough to act on it.
How the One-Hour Screening Works
The most common approach in the United States is a two-step process. The first step is the one-hour glucose challenge test, sometimes called the “glucola test.” You drink a sweet syrup containing 50 grams of sugar, then sit in the office or lab for one hour. After that hour, a blood sample is drawn from your arm.
You do not need to fast for this test. You can eat normally beforehand, and the test can be scheduled at any time of day. That makes it relatively easy to fit into a regular prenatal appointment. The drink itself is intensely sweet, similar in consistency to a flat orange soda, and some women find it unpleasant or feel a bit nauseated afterward. Drinking it cold and quickly tends to make it more tolerable.
Your provider will use a blood sugar cutoff, typically either 135 or 140 mg/dL, to determine whether you pass. If your result falls below that number, no further testing is needed. If it’s above, that doesn’t mean you have gestational diabetes. It means you need the longer, more definitive follow-up test. Roughly 15% to 20% of women who take the one-hour screen end up needing that second step.
The Three-Hour Diagnostic Test
If your one-hour result is elevated, the next step is a three-hour oral glucose tolerance test. This one does require fasting, usually for 8 to 14 hours overnight. When you arrive at the lab, a fasting blood draw is taken first. You then drink a larger glucose solution containing 100 grams of sugar. Blood is drawn again at the one-hour, two-hour, and three-hour marks, for a total of four blood draws.
This test takes the better part of a morning, so plan to bring something to read or do. You’ll need to stay at the lab or office the entire time. Many women feel more side effects from the stronger drink, including nausea, lightheadedness, or a sugar crash as the hours pass. Eating a balanced meal the night before, with protein and complex carbohydrates, can help stabilize how you feel during the test.
A gestational diabetes diagnosis requires two or more of the four blood draws to come back above the threshold values. If only one value is elevated, you typically won’t be diagnosed, though your provider may recommend dietary monitoring. Many women who are referred for the three-hour test end up passing it.
The One-Step Alternative
Some providers use a different approach: a single 75-gram, two-hour glucose tolerance test that skips the one-hour screening entirely. This method requires fasting and involves blood draws at fasting, one hour, and two hours. Unlike the two-step process, only one elevated value is needed for a gestational diabetes diagnosis.
Because the diagnostic bar is lower, this approach identifies about 18% to 20% of pregnant women as having gestational diabetes, roughly two to three times the rate of the two-step method. There is ongoing professional debate about which approach is better. In the U.S., most obstetric practices still use the two-step method, but you may encounter the one-step version depending on your provider or hospital system.
Who Gets Tested Early
While 24 to 28 weeks is the standard window, your provider may order the test in the first trimester if you carry certain risk factors. These include a body mass index over 30, a history of gestational diabetes in a previous pregnancy, a strong family history of type 2 diabetes, or a previous baby who weighed more than 9 pounds. Women with polycystic ovary syndrome or those from certain ethnic groups with higher diabetes prevalence may also be screened early. If an early test comes back normal, you’ll still be retested at the standard 24-to-28-week window.
What Happens After a Diagnosis
If the test confirms gestational diabetes, you’ll typically start by meeting with a dietitian or diabetes educator who will help you build a meal plan focused on managing carbohydrate intake. You’ll also learn to check your blood sugar at home, usually a few times a day using a finger-stick monitor. For most women, these changes are enough to keep blood sugar in a safe range.
If dietary changes alone aren’t sufficient, your provider may add medication. You’ll also have more frequent prenatal visits and possibly additional ultrasounds to monitor the baby’s growth. Delivery timing and method depend on how well blood sugar is controlled and how the baby is measuring.
After delivery, blood sugar returns to normal for most women. However, having gestational diabetes does increase your long-term risk of developing type 2 diabetes, so your provider will likely recommend a follow-up glucose test 6 to 12 weeks postpartum and periodic screening in the years that follow.