How many months into pregnancy is the glucose screening test performed?

Glucose screening, commonly referred to as the “glucose challenge test” or “gestational diabetes screening,” is typically performed between 24 and 28 weeks of gestation. This timing is evidence-based: by the late second trimester, placental hormones—such as cortisol, progesterone, and human placental lactogen—reach peak levels, inducing significant insulin resistance. This physiological change maximizes the sensitivity of the test for detecting gestational diabetes mellitus (GDM).

For individuals with high-risk factors—including pre-pregnancy obesity (BMI ≥30 kg/m²), personal history of GDM, strong family history of type 2 diabetes, polycystic ovary syndrome (PCOS), or previous delivery of a macrosomic infant (birth weight ≥4,000 g)—screening may be conducted earlier, often at the first prenatal visit. If initial fasting plasma glucose is ≥92 mg/dL (5.1 mmol/L), or HbA1c is ≥5.7%, or random glucose is ≥200 mg/dL (11.1 mmol/L), GDM may be diagnosed outright without further testing.

The standard screening protocol involves a 50-gram oral glucose challenge test (OGCT): the patient consumes a 50-g glucose solution after an overnight fast or at least 8 hours postprandial, and venous plasma glucose is measured one hour later. A result ≥140 mg/dL (7.8 mmol/L) is considered abnormal and warrants follow-up with a diagnostic 3-hour 100-gram oral glucose tolerance test (OGTT). Alternatively, some guidelines—such as those from the International Association of Diabetes and Pregnancy Study Groups (IADPSG)—recommend a one-step approach using a 75-gram OGTT with specific fasting and 1- and 2-hour thresholds.

Timely and appropriate screening is essential—not only to identify GDM but also to mitigate associated maternal and fetal risks, including preeclampsia, cesarean delivery, neonatal hypoglycemia, birth trauma, and childhood metabolic dysfunction. All pregnant individuals should discuss their individual risk profile and optimal timing for glucose screening with their obstetric provider during early prenatal care.