Background: The clinical consequences of hypothyroidism and hypothyroxinemia during pregnancy such as preterm birth are not still clear. Objective: The aim of this meta-analysis was to estimate the relation of clinical and subclinical hypothyroidism and hypothyroxinemia during pregnancy and preterm birth. Materials and Methods: In this meta-analysis, Preferred Reporting Items for Systematic review and Meta-Analysis were utilized. Searching the 83Tcohort studies83T were done by two researchers independently without any restrictions on Scopus, PubMed, Science Direct, Embase, Web of Science, CINAHL, Cochrane, EBSCO and Google Scholar databases up to 2017. The heterogeneity of the studies was checked by the Cochran's Q test and IP2P index. Both random and fixed-effects models were used for combining the relative risk and 95% confidence intervals. Data were analyzed using Comprehensive Meta-Analysis software version 2. Results: Twenty-three studies were included in the meta-analysis. The relative risks of the clinical hypothyroidism, subclinical hypothyroidism and hypothyroxinemia during pregnancy on preterm birth was estimated 1. 30 (95% CI: 1. 05-1. 61, p=0. 013, involving 20079 cases and 2452817 controls), 1. 36 (95% CI: 1. 09-1. 68, p=0. 005, involving 3580 cases and 64885 controls) and 1. 31 (95% CI: 1. 04-1. 66, p=0. 020, involving 1078 cases and 44377 controls), respectively. Conclusion: The incidence of preterm birth was higher among mothers with clinical and subclinical hypothyroidism or hypothyroxinemia during pregnancy compared to euthyroid mothers, and these relations were significant. Therefore, 83Tgynecologists and endocrinologists83T should manage these patients to control the incidence of 83Tadverse pregnancy outcomes83T such as preterm birth.