Objective To investigate the relationship between fasting plasma glucose (FPG) in early pregnancy and diagnosis of gestational diabetes mellitus (GDM) and to confirm the rationality of the new standard for GDM diagnosis in early pregnancy set by the International Association of Diabetes and Pregnancy Study Groups (IADPSG).Methods Clinical materials of 2761 pregnant women without diabetes mellitus,who accepted prenatal cares in Peking University First Hospital from April 1,2011 to December 31,2011,were collected and analyzed.The difference between FPG levels of GDM and non-GDM women was compared.According to the early pregnancy FPG level,the subjects were divided into group A (FPG<5.1 mmol/L,n=2431) and B (FPG≥5.1 mmol/L,n=330).The incidence of GDM and pregnant outcomes of the two groups were compared with t or Chi-square test.Relationship between FPG and GDM was analyzed by Logistic regression and receiver operating characteristic curve.Results (1) Among the 2761 subjects,515 were diagnosed as GDM (18.7%) and the early pregnancy FPG level in GDM group was significantly higher than that in non-GDM group [(4.84±0.46) mmol/L vs (4.57 ± 0.35) mmol/L,t =11.924,P =0.000].In early pregnancy,the risk of GDM increased by 7.984-fold (OR=8.984,95%CI:6.605-12.220) with every 1 mmol/L increase of the FPG level.(2) The diagnostic rate of GDM during mid-and last-trimester in group A (15.2%,370/2431) was lower than that of group B (43.9%,145/330),x2 =123.976,P =0.000.(3) Receiver operating characteristic curve analysis of FPG in early pregnancy and diagnosis of GDM:The largest area under the curve was 0.718 (95% CI:0.690-0.747).The sensitivity and specificity were 0.600 and 0.612,or 0.735 and 0.726 respectively,when 4.795 mmol/L or 4.785 mmol/L were set as the cut-off value.(4) Among the 1208 cases delivered,GDM was diagnosed in 227 cases.The cesarean section rate (54.2%,123/227) of GDM women was higher than that (39.2%,385/981) of non-GDM women (x2 =16.884,P=0.000).There were no differences in the incidences of macrosomia,neonatal hyperbilirubinemia,low birthweight infant,premature delivery,fetal growth restriction and preeclampsia between GDM and non-GDM group (all P>0.05).The incidence of premature birth in GDM women with FPG< 5.1 mmol/L was lower (5.8%,10/173) than that (14.8%,8/54) of women with FPG≥5.1 mmol/L (x2=4.601,P<0.05).The incidence of cesarean section,insulin administration,macrosomia and preeclampsia increased from low FPG group to high FPG group,however there was no statistical significances.Conclusions Diagnosing GDM with FPG≥5.1 mmol/L in early pregnancy is not recommended as over diagnosis might happen.But this cut-off value might indicate that the patient are at risk of GDM,and this population should not be ignored. Key words: Pregnancy trimester, first; Diabetes, gestational; Blood glucose