Objective To investigate the incidence of subclinical hypothyroidism (SCH) during the third trimester of pregnancy and its effects on pregnancy outcomes and neonatal hypothyroidism.Methods A total of 10 695 women in the third trimester of pregnancy (28-42 weeks of gestation) who labored from January 1,to December 31,2012 in Changzhou Maternal and Child Health Care Hospital Affiliated to Nanjing Medical University were enrolled.The levels of thyroid stimulating hormone (TSH),free thyroxine (FT4) and thyroid peroxidase antibody (TPOAb) of the mothers were quantified by electrochemical immunoassay.The time-resolved fluorescence immunoassay was used to detect neonatal thyroid hormone levels.Using t test or x2test,the incidence of adverse pregnancy outcomes was compared between SCH group and euthyroid group and between SCH women with positive (n=40) or negative TPOAb (n=176).Results The incidence of SCH was 2.02％ (216/10 695) and the positive rate of TPOAb in SCH women was 18.5％ (40/216).No neonatal thyroid dysfunction was found.According to the age matched,222 cases were randomly selected as controls from 7 757 euthyroid women.Compared with the controls,SCH women had a higher incidence of premature rupture of membranes [28.7％ (62/216) vs 14.9％ (33/222),x2=12.34],anemia [11.6％ (25/216) vs 4.1％ (9/222),x2=8.65],pregnancy-induced hypertension [9.7％ (21/216) vs 4.5％ (10/222),x2=4.53],premature labor [8.8％ (19/216) vs 3.6％ (8/222),x2=5.10] and intrahepatic cholestasis of pregnancy [8.3％(18/216) vs 2.3％ (5/222),x2=8.14] (all P＜0.05).The overall incidence of adverse pregnancy outcomes was also higher in SCH group than in the controls [69.4％ (150/216) vs 49.5％ (110/222),x2=17.96,P＜0.01].The incidence of fetal growth restriction and still birth in SCH mothers with positive TPOAb was higher than in those with negative TPOAb [7.5％ (3/40) vs 0.0％ (0/176),x2=13.32,P＜0.01; 2.5％ (1/40) vs 0.0％ (0/176),x2=4.40,P＜0.05],but there was no significant difference in the overall incidence of adverse pregnancy outcomes compared with TPOAb-negative mothers [65.0％ (26/40) vs 70.5％ (124/176),x2=0.46,P=0.50].Conclusions SCH diagnosed in the third trimester may lead to adverse pregnancy outcomes.Early screening for thyroid dysfunction is necessary.