When is gestational diabetes mellitus screening usually performed?

• A. First trimester
• B. Between 24–28 weeks gestation
• C. After 36 weeks
• D. Between 24–28 weeks gestation (with diagnostic testing if screening is positive)

Answer: (D)

Screening for gestational diabetes is timed to mid-pregnancy, when insulin resistance from placental hormones tends to become evident, making it the right window to identify new glucose intolerance without picking up most preexisting diabetes. That’s why the typical approach is to screen between 24 and 28 weeks. If the screening test is positive, a diagnostic test is then done to confirm the diagnosis and guide management.

Early in the first trimester, testing mainly helps identify preexisting diabetes, not gestational diabetes that develops later. Testing after 36 weeks is unlikely to change management or outcomes. So the best approach combines screening in the 24–28 week window with diagnostic testing if the screen is positive.