The pregnancy glucose screening is a routine test, generally performed between the 24th and 28th weeks of gestation, to identify gestational diabetes mellitus (GDM). This form of diabetes develops during pregnancy and can pose health risks to both the parent and the developing fetus if left unmanaged. Accurate test results are important for timely intervention, which typically involves diet and lifestyle changes to maintain healthy blood sugar levels. Proper preparation, particularly regarding food intake, directly influences the reliability of the outcome.
Dietary Preparation in the Days Before
The goal of pre-test preparation is to allow your body’s typical glucose processing ability to be measured, not to artificially manipulate the result. Maintaining regular eating habits in the days leading up to the test is the standard recommendation. Providers generally advise against drastically restricting carbohydrates, as a low-carb diet can temporarily alter the body’s metabolic response to the glucose drink, potentially skewing the results.
For the diagnostic 3-hour glucose tolerance test, some protocols recommend consuming a diet containing at least 150 grams of carbohydrates daily for a minimum of three days before the test. This practice ensures that the insulin-producing cells of the pancreas are adequately stimulated before the diagnostic challenge. Avoiding crash diets or unusual eating patterns remains consistent advice, even if specific carbohydrate loading is not mandatory for everyone. Consistency is key, ensuring your diet reflects your normal intake so the test accurately reflects your body’s ability to process sugar.
Eating and Fasting Guidelines for Test Day
Preparation instructions differ significantly depending on whether you are undergoing the initial 1-hour glucose screen or the follow-up 3-hour diagnostic test. For the common 1-hour glucose challenge test, which involves consuming a 50-gram glucose solution, complete fasting is typically not required. Many healthcare providers permit a small meal in the hours before the appointment to prevent nausea and lightheadedness.
If you choose to eat before the 1-hour screen, the meal should be light, focused on protein and complex carbohydrates, and consumed one to two hours before the test. Excellent choices include eggs, whole-grain toast, or plain yogurt, as these foods help stabilize blood sugar levels. You must strictly avoid simple sugars and high-glycemic foods, such as fruit juices, sugary cereals, pancakes, and pastries, which can cause an exaggerated glucose spike and lead to a false positive result.
The 3-hour oral glucose tolerance test (OGTT), however, requires absolute fasting, making the preparation stricter. You must not consume food or drink, other than small sips of plain water, for eight to fourteen hours before the initial blood draw. This ensures the fasting blood sample accurately reflects baseline glucose levels without recent dietary influence. The test procedure begins with a fasting blood draw, followed by consuming a 100-gram glucose solution, and then three more blood draws taken one, two, and three hours after finishing the drink. Because protocols can vary, confirm the exact fasting requirements with your healthcare provider or the testing facility.
Non-Dietary Factors That Influence Accuracy
Beyond food and drink, several other factors can impact the accuracy of your glucose test results. Certain medications, including corticosteroids and some diuretics, can cause blood sugar levels to rise by interfering with insulin sensitivity or glucose production. Discuss all prescription and over-the-counter medications with your provider before the test, as they may advise a temporary pause or adjustment.
Acute illness or stress can also raise blood glucose levels, potentially leading to an inaccurate result. When the body is fighting an infection or under physical stress, hormones like cortisol are released, which naturally increase blood sugar. If you are unwell on the day of the test, it may be advisable to reschedule. Vigorous exercise should also be avoided in the 24 hours leading up to the test, as intense physical activity can affect how the body metabolizes glucose.
Interpreting Initial Results and Follow-Up
A result below the established threshold on the initial 1-hour glucose screen is considered normal, indicating that your body processed the 50-gram glucose load effectively. The most common cutoff value used to determine if follow-up testing is needed is 140 milligrams per deciliter (mg/dL) one hour after drinking the solution. Some providers use a more conservative threshold of 130 mg/dL to capture more cases.
If your result is at or above the cutoff, it does not mean you have gestational diabetes; it simply signals that further evaluation is required. The next step is the 3-hour oral glucose tolerance test, a diagnostic procedure involving four separate blood draws. A diagnosis of GDM is confirmed if two or more of the four glucose values from the 3-hour test meet or exceed the specific criteria:
- Fasting at 95 mg/dL.
- 1 hour at 180 mg/dL.
- 2 hours at 155 mg/dL.
- 3 hours at 140 mg/dL.
If only one value is elevated, you may be asked to repeat the test later in the pregnancy.