Abstract： Objective To investigate the prevalence of hypothyroidism during the second trimester and its relationship with thyroid peroxidase antibody (TPOAb). Methods Two thousand one hundred and forty one pregnant women whose gestational age between 14 to 28 weeks,accepted their prenatal care at the outpatient clinic of International Peace Maternity & Child Health Hospital from March 1,2010 to July 31,2010 were enrolled into this study. Serum TPOAb,thyroidstimulating hormone (TSH) and free thyroxine (FT4) of these women were detected. Logistic regression model was used to analyze the risk factors of subclinical hypothyroidism and positive TPOAb,while Spearman rank correlation was used to analyze the relationship between the levels of TSH,FT4 and TPOAb. Results (1) Subclinical hypothyroidism was detected in 13.36％ (286/2141) patients. Isolated maternal hypothyroxinemia occurs in 0.14％ (3/2141) of pregnant women.No overt hypothyroidism patient was detected and 6.26％ (134/2141) of all pregnant women exhibited positive TPOAb(≥50 U/ml).(2) Positive rate of TPOAb in subclinical hypothyroidism group,isolated maternal hypothyroxinemia group and normal thyroid function group was 13.64％ (39/286),0/3 and 5.06％ (86/1701) respectively,and there was difference among the three groups (x2 =30.82,P＜0.01).The positive rate of TPOAb did not relate to fetal gender,maternal age,gestational age,gravidity and parity.(3) TPOAb had positive relationship with TSH level (r=0.12,P＜0.01),while did not relate to FT4 level (r=-0.04,P=0.09). (4) Positive TPOAb (OR 3.18,95％ CI:2.10-4.83,P＜0.01) and gravidity (OR=1.21,95％ CI:1.02-1.43,P=0.030)were risk factors of subclinical hypothyroidism. Conclusions Hypothyroidism is common during the second trimester. It is necessary to screen TPOAb in pregnant women as TPOAb is an independent and important predictor of subclinical hypothyroidism.