What screening test is commonly used for gestational diabetes between 24-28 weeks, and what follow-up test is used if positive?

• A. 50 g glucose challenge test; if abnormal, 3-hour 100 g oral glucose tolerance test
• B. HbA1c test
• C. 75 g two-hour glucose tolerance test
• D. Urine glucose testing

Answer: (A)

Screening for gestational diabetes during the second trimester uses a nonfasting glucose challenge test: you give a 50 g glucose drink and measure the blood glucose after one hour. If this screening value is above the established threshold (typically about 130–140 mg/dL, depending on the protocol), a diagnostic follow-up test is performed. That follow-up test is a 3-hour 100 g oral glucose tolerance test, done after fasting, with glucose measurements at fasting and at 1, 2, and 3 hours. If enough values exceed the cutoffs, gestational diabetes is diagnosed.

HbA1c isn’t reliable for screening in pregnancy because it can miss cases and doesn’t reflect the postabsorptive glucose fluctuations essential to diagnosing GDM. Urine glucose testing is not sensitive or specific enough to diagnose GDM. The 75 g two-hour OGTT is used in some one-step diagnostic approaches, but the standard two-step screening-and-diagnosis sequence in many practice settings starts with the 50 g glucose challenge test followed by the 3-hour 100 g OGTT if positive.