Screening for gestational diabetes mellitus (GDM) is typically performed at 24–28 weeks gestation because this timing corresponds to the period when placental hormones cause increased insulin resistance. Early screening is reserved for high-risk women, but the majority develop glucose intolerance in the second trimester. Testing at this stage allows for timely diagnosis and management, reducing risks of adverse maternal and fetal outcomes. Screening earlier or later is less effective in identifying most cases.

Reference: Williams Obstetrics, Cunningham et al, 25th Edition.