Failing the 1-hour glucose challenge test is a moment of concern for many pregnant people. This test, often called the glucose challenge test (GCT), is a standard, non-fasting screening procedure typically administered between the 24th and 28th weeks of pregnancy to check for gestational diabetes (GD). The result is not a diagnosis, but rather an indication that your body needs a more detailed assessment of how it manages sugar. A significant number of people who fail this initial screen do not ultimately have gestational diabetes, as the screening is intentionally designed to be highly sensitive to ensure no potential cases of GD are missed.
Interpreting the 1-Hour Screening Results
The 1-hour glucose challenge test involves consuming a 50-gram glucose drink, followed by a single blood draw an hour later. The result is interpreted against a specific blood sugar threshold. Common cutoff thresholds for a “failed” screening result are typically set at 130 mg/dL or 140 mg/dL. This screening approach is designed for high sensitivity, aiming to catch nearly all cases of gestational diabetes, even at the cost of a high rate of false-positive results. The test is non-fasting, which contributes to result variability, as recent meals can influence the final blood sugar level. Approximately 15% to 20% of pregnant women will fail the initial 1-hour test and be asked to proceed to the diagnostic test. The elevated result suggests that your body’s initial response to the concentrated glucose load was slower or less effective than the set threshold, prompting a more definitive, fasting evaluation.
Preparing for the Diagnostic 3-Hour Test
A failed 1-hour screening test requires proceeding to the 3-hour, 100-gram Oral Glucose Tolerance Test (OGTT), which is the definitive diagnostic procedure. This test requires careful preparation to ensure the results are accurate by stabilizing your body’s glucose metabolism beforehand.
For three days before the test, you must consume an unrestricted diet that includes at least 150 grams of carbohydrates daily. This high-carbohydrate intake is necessary to ensure your pancreas is actively producing insulin and is not falsely suppressed by a low-carb diet.
The night before the test, you must fast for 8 to 14 hours, consuming only water. It is also important to avoid smoking, heavy exercise, and certain medications, such as corticosteroids, as these factors can interfere with accurate glucose metabolism.
The diagnostic procedure involves an initial fasting blood draw, followed by consuming a 100-gram glucose solution, which is significantly more concentrated than the screening drink. After consuming the drink, blood samples are drawn exactly at the one-hour, two-hour, and three-hour marks, for a total of four blood draws. You must remain seated and refrain from eating, drinking anything other than water, or performing strenuous activity during the entire three-hour test duration.
Understanding the Diagnostic Criteria
The results from the four blood draws of the 3-hour OGTT are compared against established diagnostic criteria to confirm or rule out gestational diabetes. Two main sets of thresholds are commonly used in the United States: the National Diabetes Data Group (NDDG) criteria and the Carpenter-Coustan criteria. The Carpenter-Coustan thresholds are lower and therefore more inclusive, leading to a higher rate of diagnosis.
Diagnosis of gestational diabetes is confirmed only if two or more of the four glucose values meet or exceed the established cutoffs for the specific criteria used by your provider. For example, the Carpenter-Coustan criteria set the thresholds at 95 mg/dL for the fasting draw, 180 mg/dL at one hour, 155 mg/dL at two hours, and 140 mg/dL at three hours. If only one value is elevated, the result is technically considered normal, although this may prompt increased monitoring due to the possibility of borderline glucose intolerance.
If the diagnosis of GD is confirmed, the next steps focus on careful management, typically beginning with dietary modifications and a structured monitoring schedule. The goal is to keep blood sugar levels within a healthy range, often targeting a fasting level below 95 mg/dL and a one-hour post-meal level below 140 mg/dL. This management is aimed at reducing risks for both the pregnant person and the baby, such as the risk of the baby growing too large (macrosomia).