The gestational diabetes screening is a routine part of prenatal care, typically performed between the 24th and 28th weeks of pregnancy to check how the body processes sugar. The initial step is often a glucose challenge test (GCT); if the result is abnormal, a diagnostic oral glucose tolerance test (OGTT) is required to confirm a diagnosis. Proper preparation for the diagnostic OGTT is important because the test measures the body’s ability to handle a significant glucose load. Inaccurate preparation, especially regarding diet, fasting, or medication use, can lead to skewed results, potentially causing a false positive or requiring the test to be repeated.
The Three-Day Prep: Ensuring Adequate Carbohydrate Intake
For the three days leading up to the diagnostic OGTT, a consistent and adequate intake of carbohydrates is necessary. Healthcare providers typically advise consuming at least 150 grams of carbohydrates daily during this period. This carbohydrate-loading diet ensures the body’s glucose-processing system is functioning normally before the test.
A low-carbohydrate diet before the test can sometimes lead to a false positive result. If the body is unaccustomed to processing carbohydrates, the liver may overcompensate by increasing glucose production, causing an artificially elevated blood sugar reading. To meet the 150-gram target, include carbohydrate sources in all three major meals and incorporate them into snacks.
Acceptable carbohydrate sources include starches like bread, pasta, rice, and potatoes, as well as fruits and certain dairy products like yogurt. For example, a single cup of cooked pasta or rice provides around 45 grams of carbohydrates. Consuming a regular, balanced diet that is not restricted in carbohydrates ensures the test accurately reflects the body’s true metabolic state.
The Critical 12 Hours: Fasting and Restrictions
The diagnostic oral glucose tolerance test requires a strict period of fasting immediately beforehand to establish a baseline blood glucose level. This fasting window is usually set for a minimum of 8 hours and a maximum of 14 hours before the first blood draw. The most common instruction is to stop all food and drink, except for plain water, after midnight the night before a morning appointment.
Fasting means no consumption of food, including small snacks, and no caloric beverages like juice, soda, or coffee, even if they are sugar-free or diet versions. Non-food items that can interfere with glucose metabolism or contain hidden sugars must also be avoided, such as chewing gum, mints, cough drops, and hard candy. Consuming any of these items can instantly invalidate the fasting blood draw and require the appointment to be rescheduled.
Plain water is the only exception allowed and is encouraged to maintain hydration, which can make the blood draw process easier. During the multi-hour test, which typically involves four blood draws over three hours, no food or drink, other than small sips of water, is permitted. Physical activity should also be kept to a minimum, as exercise can affect glucose levels.
Medications and Supplements: What to Discuss with Your Provider
Certain non-dietary substances can significantly interfere with the body’s glucose processing and the accuracy of the test results. Prescription medications are a primary concern, especially corticosteroids, which are known to increase blood sugar levels. Other medications, including certain diuretics and some drugs used for anxiety or depression, can also impact glucose metabolism.
Over-the-counter (OTC) medications and supplements also require discussion with a healthcare provider before the test. High doses of Vitamin C can affect glucose measurements, while certain cough syrups or decongestants like pseudoephedrine may elevate blood sugar. Even common antibiotics, such as those in the penicillin or cephalosporin families, have been noted to cause falsely elevated glucose readings.
The most effective preparation is to compile a complete list of all prescriptions, OTC drugs, vitamins, and herbal supplements currently being taken. The healthcare provider can then advise whether any of these substances should be temporarily paused or adjusted in the 24 hours leading up to the test. Never stop taking a prescription medication without first consulting the prescribing clinician.