To determine the effect of changes in the diagnostic criteria on the number of gestational diabetes mellitus (GDM) detected and on pregnancy and neonatal outcomes. Methods: We retrospectively reviewed results of the 75g oral glucose tolerance test and pregnancy and neonatal outcomes of Chinese women with singleton pregnancies delivered at Tuen Mun Hospital between January and December 2016. Those with GDM was treated with lifestyle modification with or without insulin. Women with GDM detected

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... the old and new criteria were compared in terms of the numbers of GDM detected, maternal characteristics, pregnancy outcomes, and neonatal outcomes. Results: Of 733 pregnant women, 211 (28.8%) and 190 (25.9%) were identified as having GDM based on the old or new criteria, respectively (p=0.01). Women with GDM based on the old or new criteria were comparable in terms of maternal characteristics, pregnancy outcomes, and neonatal outcomes. Among the 190 women with GDM based on the new criteria, 33 (17.4%) had normal fasting blood glucose and 2-hour glucose results but abnormal 1-hour glucose result. Compared with women without GDM, women with GDM detected by 1-hour glucose test alone had lower birthweight neonates (3.04 kg vs 3.22 kg, p=0.01), more neonates small for gestational age (3.7% vs 15.2%, p=0.01), with hypoglycaemia (15.2% vs 3.9%, p<0.001), and admission to neonatal intensive care unit (12.1% vs 1.3%, p<0.001). Conclusions: The new criteria detected 2.9% fewer women with GDM. 17.4% of women with GDM who were associated with poor neonatal outcomes were detected exclusively by 1-hour glucose test. The new criteria can help identify high-risk women for fetal monitoring.