Objective To observe and compare the variation of insulin resistance and pancreatic β-cell function in pregnant women with abnormal and normal glucose metabolism during the first,second,and third trimesters,and to explore the feasibility of early diagnosis of gestational diabetes mellitus(GDM).Methods This is a prospective study.507 pregnant women with regular antenatal care from February 2009 to March 2010 were included in the study.Based on the results of oral glucose tolerance test,the patients with GDM consisted of 58,86,and 66 subjects respectively in the first,second,and third trimesters.The control group included 72,164,and 66 subjects respectively in the first,second,and third trimesters.Homeostasis model assessment insulin resistance index( HOMA-IR),homeostasis model assessment β-cell function (HOMA-β),area under curve of glucose (AUCG),area under curve of insulin ( AUCI ),30-minute insulin increase to 30-minute glucose increase ( △I30/△G30 ),and insulin sensitivity index composite(ISIcomp) were calculated for the evaluation of insulin resistance and pancreatic β-cell function.Results ( 1 ) AUCG and AUCI in the GDM group were statistically higher than those in control group while △I30/△G30and ISlcomp in the GDM group were statistically lower than those in the control group during the first trimester(P＜0.05),but HOMA-IR showed no statistically significant difference between two groups.In the GDM groups HOMA-IR,AUCG,and AUCI were statistically higher,whereas △I30/△G30 and ISIcomp were statistically lower than those in the control groups during the second and third trimesters (P ＜ 0.05 ). HOMA-β showed no statistically significant differences between the GDM groups and control groups during 3 trimesters.( 2 ) During thefirst,second,and third trimesters,blood glucose reached its peak by 0.5 h in the oral glucose tolerance test,and insulin reached its peak by 0.5-1 h in the control groups; while in the GDM groups the respective figures were 1 h and 2 h.Conclusions The insulin resistance in GDM patients diagnosed during the first,second,and third trimesters was higher than in pregnant women with normal glucose metabolism,and the peaks of blood glucose and insulin reached in oral glucose tolerance test were respectively delayed.The impaired pancreatic β-cell insulin secretion in GDM patients was present from early pregnancy.Thus for high-risk groups,oral glucose tolerance test during early pregnancy will be helpful for screening abnormal glucose metabolism.