Diagnostic criteria of gestational diabetes mellitus: (usually performed at 24-28 weeks of gestation) 1. GDM can be diagnosed if fasting blood glucose is ≥5.1 mmol/L; GDM can be diagnosed if fasting blood glucose is ≥4.4 mmol/L but <5.1 mmol/L. The second step of 75g OGTT should be performed if the result is abnormal. 2. Normal values of 75g OGTT: fasting, 1h and 2h blood glucose values after taking glucose are less than 5.1 mmol/L, 10.0 mmol/8.5 mmol/L respectively. Any point with abnormal blood glucose value should be diagnosed as GDM. Glucose tolerance test (OGTT) test method: first measure fasting blood glucose, then take 75g of anhydrous glucose orally (dissolved in 300ml water, finished within 5min). Then measure the intravenous blood glucose 1h and 2h after taking sugar respectively (the time is calculated from the first sip of sugar water).