Background: Gestational diabetes mellitus (GDM), defined as hyperglycaemia first identified during pregnancy, is associated with increased risks of adverse maternal and neonatal outcomes. Standard screening is typically performed at 24–28 weeks’ gestation; however, earlier screening before 20 weeks has been proposed to improve outcomes. Evidence supporting early screening or diagnosis remains inconsistent. We aimed to evaluate whether early screening or early diagnosis of GDM (before 20 weeks) influences maternal and neonatal outcomes compared with standard screening or diagnosis at 24–28 weeks.Methods: We conducted a systematic review and meta-analysis of analytical studies comparing early versus standard timing of GDM screening or diagnosis. A systematic search of PubMed, Embase, Web of Science, and Cochrane CENTRAL was performed from inception to 2025. Eligible studies included those comparing early versus usual screening in unselected pregnant populations and/or early versus usual diagnosis among women with GDM; these comparisons were carefully predefined to minimize potential bias. Two reviewers independently screened studies, extracted data, and assessed methodological quality using the MASTER scale. Pooled effects were generated using the quality-effects model. Publication bias was assessed using Doi plots. For binary outcomes, odds ratios were calculated and converted to absolute measures (risk differences and numbers needed to treat or harm) across a range of baseline risks to enhance interpretability. Outcomes assessed included gestational weight gain, preterm delivery, preeclampsia or eclampsia, macrosomia, neonatal hypoglycemia, neonatal intensive care unit admission, large-for-gestational-age infants, and caesarean delivery.Finding:14 studies met the inclusion criteria. Early diagnosis was not associated with a statistically divergent difference from usual diagnosis in terms of pregnancy-weight gain (WMD -1.08 kg), preterm delivery (OR 1.09), preeclampsia/eclampsia (OR 1.81), macrosomia (OR 0.97), PIH (OR 1.06), neonatal hypoglycemia (OR 1.61), NICU admission (OR 1.27), LGA (OR 0.98) and cesarean section (OR 1.18). Across the general obstetric population, early screening did not demonstrate a statistically divergent difference in any maternal or neonatal outcome including preterm delivery (OR 1.54), preeclampsia/eclampsia (OR 1.24), neonatal hypoglycemia (OR 1.19), Cesarean section (OR 0.87), macrosomia (OR 0.96) and LGA (OR 1.15). Translation of relative effects into absolute measures indicated no absolute clinical benefit for either early diagnosis or early screening. For early diagnosis, absolute risk differences were generally small and baseline-dependent, with directionally harmful effects for only few outcomes, particularly preeclampsia/eclampsia and neonatal hypoglycemia (NNH = 7 to 20), whereas most other outcomes demonstrated risk differences close to zero. Additionally, for early screening, absolute risk differences were largely neutral, with only preterm birth showing a small increase in absolute risk and all other outcomes indicating negligible differences.Interpretation: Early diagnosis of GDM does not identify a group with substantially different maternal and neonatal risks, likely reflecting a population for which intervention is unnecessary. Early screening, when applied to the general population of pregnant women, also does not improve outcomes and offers no absolute interventional benefit. Current evidence does