The Relationship of Maternal Subclinical Hypothyroidism during Pregnancy and Preterm Birth: A Systematic Review and Meta-Analysis of Cohort Studies

The relationship between maternal subclinical hypothyroidism during pregnancy and preterm birth is still not clear and the results of previous studies have been inconsistently reported. This study was performed with aim to assess the relationship between mother's subclinical hypothyroidism during pregnancy and preterm.
This study was conducted based on preferred reporting items for systematic reviews and meta-analyses (PRISMA) checklist. Review and searching of the literatures were done by two researchers independently in databases such as Scopus, PubMed, ScienceDirect, Embase, Springer, Web of Science, CINAHL, Wiley Online Library, Cochrane Library, EBSCO and Google scholar till June 2017 with standard English MeSH keywords including Thyroid Disease, Hypothyroidism, Subclinical Hypothyroidism, Preterm Delivery, Preterm Labor, Preterm Birth, Premature Delivery, Premature Labor and Premature Birth. Random-effects size based on Cochrane test and I2 were used to pool relative risk (RR) and estimate 95% Confidence Interval (CI). Data were analyzed by Comprehensive Meta-Analysis software ver. 2 software.
Seventeen studies with 3,580 experimental and 64,885 control sample size were included. The RR of preterm birth for pregnant women with subclinical hypothyroidism compared with health group was 1.36 (95% CI: 1.09-1.68, P=0.005) and was statistically significant. RR for this relationship in Asian studies was estimated 1.60 (95% CI: 1.12-2.29, P=0.009) and was statistically significant, but this significant relationship was not found in American (P=0.576) and European (P=0.072) studies.
The incidence of preterm birth for pregnant women with subclinical hypothyroidism was higher compared to euthyroid pregnant women and this relationship was significant. Therefore, physicians should consider the management of these patients in terms of adverse pregnancy outcomes such as preterm birth.