Background:Adverse maternal outcomes and perinatal complications are closely associated with overt maternal hypothyroidism, but whether these complications occur in women with subclinical hypothyroidism (SCH) during pregnancy remains controversial.The aim of this study was to evaluate the effects of SCH on maternal and perinatal outcomes during pregnancy. Methods:This retrospective cohort study was conducted by examining the pregnant women who applied to the Gynaecology Department of Hind Institute of Medical Sciences and Hospital at their 6th to 14th gestational weeks and had antenatal follow-ups between February 1, 2021 and December 31, 2022. Results: The TSH concentration was significantly lower in the first trimester than in the third trimester (P,0.001). The fT4 concentration was higher in the first trimester than in the second and third trimesters (P,0.001). The incidences of GH and PROM were significantly higher in women with SCH than in euthyroid women (5% vs. 1.57%, P= 0.020 10% vs. 5%, P= 0.002).IUGR was more frequent in women with SCH than in euthyroid women (7.5% vs. 1.3%, P,0.001). More LBW infants were delivered in the SCH group than in the euthyroid group (12.5% vs. 5%, P,0.001). Conclusion:The results of this study indicate that pregnant women with SCH had increased risks of GH and PROM, and their foetuses and infants had increased risks of IUGR and LBW. Thus, routine maternal thyroid function testing is necessary to improve maternal and perinatal outcomes.