The glucose screening test, often called the Glucose Challenge Test (GCT) or the full Glucose Tolerance Test (GTT), is a standard procedure in pregnancy designed to detect Gestational Diabetes Mellitus (GDM). GDM involves elevated blood sugar levels that develop during pregnancy, which can pose risks to both the expectant mother and the baby. Because this test measures the body’s ability to process sugar, proper preparation is key to ensuring the results accurately reflect your metabolic function. Testing protocols can vary, and you must always follow the specific instructions provided by your healthcare provider.
Dietary Preparation: The Days Before
The days leading up to your glucose test are crucial for priming your body to metabolize sugar correctly and preventing a misleading result. A common misunderstanding is that restricting carbohydrates before the test will help “pass” it, but this dietary change can actually cause an inaccurate result. When the body is deprived of its usual carbohydrate intake, it can temporarily impair glucose processing, leading to a falsely elevated reading when the high-sugar test drink is consumed.
Experts recommend maintaining a normal or slightly higher carbohydrate intake for two to three days before the test. You should aim to consume a minimum of 150 grams of carbohydrates each day during this preparatory period. This ensures your body’s insulin response mechanisms are active and functioning under typical conditions.
Focus on incorporating healthy, complex carbohydrate sources into your meals, such as whole-grain bread and pasta, brown rice, starchy vegetables like potatoes and corn, and whole fruits. Maintaining your usual level of hydration with plain water is also important during this time to support overall metabolic function.
The Essential Overnight Fast
The final hours before the test are the most critical phase, especially if you are undergoing the multi-hour Glucose Tolerance Test (GTT), which requires a fasting blood draw. You will be instructed to fast for a specific period, typically between 8 and 14 hours, before your first blood sample is taken. This means absolutely no food or caloric beverages are permitted after a certain time, often after midnight.
During this mandatory fasting window, only plain water is allowed to maintain hydration. You must strictly avoid anything that contains calories or stimulates the digestive system, including gum, mints, cough drops, or hard candies. Even seemingly insignificant items can trigger a metabolic response that invalidates the fasting blood sugar level.
It is also necessary to abstain from smoking, as nicotine can affect glucose metabolism, and strenuous exercise, which can alter blood sugar levels. If you take prescription medications, you must discuss them with your healthcare provider beforehand. Your doctor will advise if you can take necessary medications with a small sip of water during the fast, or if they need to be temporarily adjusted. Following these specific instructions ensures the accuracy of the baseline fasting measurement.
What to Expect and Post-Test Recovery
On the day of the test, after your initial fasting blood draw, you will be given a measured glucose solution to drink, usually containing 75 or 100 grams of glucose, which must be consumed quickly. You will then be required to remain seated quietly in the testing area for the duration of the procedure to prevent physical activity from artificially lowering your blood sugar. The multi-hour test involves blood samples being taken at timed intervals—typically at one, two, and three hours after you finish the drink.
To help pass the time, it is advisable to bring a book, phone, or other distraction material. Wearing comfortable, loose-fitting clothing is also recommended, as you will be seated for an extended period. Once the final blood sample has been taken, the test procedure is complete.
The high concentration of sugar in the test drink, followed by the rapid drop in blood sugar afterward, can often cause feelings of nausea, dizziness, or lightheadedness. To counteract this “sugar crash” and stabilize your blood sugar, you should eat a balanced meal immediately following the last blood draw. This recovery meal should include a combination of protein, fiber, and complex carbohydrates to provide sustained energy and prevent further discomfort. Examples include eggs with whole-grain toast or a sandwich with lean protein and vegetables.