Whether you need to fast depends on which gestational diabetes test you’re taking. The initial screening, called the 1-hour glucose challenge test, does not require fasting. The follow-up diagnostic test, called the 3-hour glucose tolerance test, requires you to fast for at least eight hours beforehand. Most women only take the first test, so there’s a good chance you can eat normally before your appointment.
The 1-Hour Screening: No Fasting Needed
The first test most pregnant women encounter is the 1-hour glucose challenge. It’s a screening, not a diagnosis. You drink a bottle containing 50 grams of a sugary glucose beverage (finishing it within five minutes), then sit in the office or lab for one hour before having your blood drawn. You can eat and drink normally before this test, and many providers even encourage you to eat a regular meal a couple of hours beforehand so you feel comfortable.
This screening typically happens between 24 and 28 weeks of pregnancy. Your provider may schedule it earlier if you’ve had high glucose in your urine at a routine prenatal visit or if you have risk factors for diabetes, such as a family history, a previous pregnancy with gestational diabetes, or a higher BMI.
The 3-Hour Diagnostic Test: Fasting Is Required
If your 1-hour screening comes back elevated, your provider will schedule a 3-hour glucose tolerance test to confirm or rule out gestational diabetes. This one does require fasting. You’ll need to avoid all food and drinks (except plain water) for at least eight hours before the test, which is why most labs schedule it first thing in the morning.
The 3-hour test involves four blood draws total. The first is a fasting blood sample taken before you drink anything. Then you drink a larger glucose solution (100 grams instead of 50), and your blood is drawn again at the one-hour, two-hour, and three-hour marks. You’ll need to stay at the lab or clinic for the full three hours, so bring something to read or watch.
A fasting baseline is essential for the diagnostic test because it establishes how your body handles sugar at rest before introducing a controlled glucose load. The pattern of how your blood sugar rises and falls over three hours reveals whether your body is producing enough insulin to keep up with the demands of pregnancy.
How to Prepare for Each Test
For the 1-hour screening, no special preparation is needed. Eat your normal meals. Some women find the glucose drink easier to tolerate if they’ve had a light meal with some protein an hour or two before, rather than drinking it on a completely empty stomach.
The 3-hour test takes more planning. In the three days leading up to it, aim to eat at least 150 grams of carbohydrates per day. That’s roughly the equivalent of eating your usual bread, pasta, rice, fruit, and starchy foods without cutting back. This matters because restricting carbs in the days before the test can actually skew your results, making your body less efficient at processing the glucose load and potentially triggering a false abnormal reading. Then, the night before your test, stop eating after dinner and fast through the morning until your appointment.
The glucose drink itself can cause nausea, lightheadedness, or bloating in some women, especially on an empty stomach during the 3-hour version. These side effects are temporary and usually pass within the first hour. Sipping cold water and sitting comfortably can help.
What Happens If You Fail the 1-Hour Test
An elevated result on the 1-hour screening doesn’t mean you have gestational diabetes. It means your blood sugar was above the cutoff after the glucose challenge, and you need the longer test to find out what’s really going on. At the standard cutoff of 140 mg/dL, roughly 65% of women who fail the screening go on to pass the 3-hour diagnostic test. The higher your 1-hour number, the more likely the diagnostic test will confirm gestational diabetes, but plenty of women with moderately elevated screenings end up with completely normal results on the follow-up.
How Gestational Diabetes Is Diagnosed
During the 3-hour test, your blood sugar is checked at four points: fasting, one hour, two hours, and three hours after drinking the glucose solution. Each time point has its own threshold:
- Fasting: 95 mg/dL or higher
- 1 hour: 180 mg/dL or higher
- 2 hours: 155 mg/dL or higher
- 3 hours: 140 mg/dL or higher
You need two or more of these values to come back elevated for a gestational diabetes diagnosis. If only one value is abnormal, you typically won’t be diagnosed, though your provider may recommend monitoring your diet or retesting later in pregnancy. These thresholds are based on the Carpenter-Coustan criteria used by the American College of Obstetricians and Gynecologists, though some practices use slightly different cutoffs, so your specific numbers may vary by a few points.
Why the Test Happens When It Does
Gestational diabetes develops because of hormonal changes in the second half of pregnancy. The placenta produces hormones that progressively reduce your body’s sensitivity to insulin, which is the hormone responsible for moving sugar out of your blood and into your cells. For most women, the pancreas compensates by producing more insulin. In women who develop gestational diabetes, the pancreas can’t keep up with the increased demand, and blood sugar starts climbing higher than normal after meals.
This insulin resistance ramps up most significantly between 24 and 28 weeks, which is why that window is the standard screening period. Testing too early can miss cases that haven’t developed yet, while testing too late reduces the time available to manage blood sugar and protect the baby from complications of prolonged high glucose exposure.