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Background: Preterm labour prediction has been relied on history of previous preterm birth and cervical length of current pregnancy. However, universal cervical length measurement has some limitation. We aim to find a surrogate marker of cervical length to close the gap in preterm prevention program and lower uterine wall thickness seems promising. We generate the nomogram of lower uterine wall thickness during 18-22 weeks of gestation and evaluate the accuracy of LUW thickness as a predictor of preterm delivery before 37 weeks.Methods: This prospective cohort study included 524 Thai singleton pregnant women at 18-22 weeks of gestation between November 2016 and October 2017. After signing informed consent, transabdominal ultrasonography was performed to examine fetal anatomical structures and to measure LUW thickness. The results were blinded to the caregivers. The outcomes of all pregnancies were followed. The LUW thickness at 10th percentiles was established and was correlated with the outcomes of pregnancy. The performance of LUW thickness at 10th percentile as a predictor of preterm delivery was calculated. The intra-observer and inter-observer reliability of measurement were assessed by intraclass correlation coefficient and Bland-Altman plot.Results: Of the 524 pregnant women, 64 (12.2%) delivered before 37 weeks of gestation. The reference value of lower uterine wall thickness at 18-22 weeks was established. Mean and 10th percentile of LUW thickness were 6.2 and 4.5 mm respectively. The inter-observer and intra-observer variation of measurement were small (intraclass correlation coefficient = 0.926 and 0.989 respectively). Using LUW thickness at less than 4.5 mm as a predictor of preterm delivery, we found a 2.37 folds increased risk of preterm delivery after adjustment of other factors (p = 0.037). Sensitivity, specificity, positive predictive value and negative predictive value were 14% (95% CI: 6.64-25.02), 92.8% (95% CI: 90.06-95.12), 22.5% (95% CI: 12.66-36.76) and 88% (95% CI: 86.92-89.08) respectively.Conclusions: The measurement of LUW thickness by transabdominal ultrasonography is feasible and reproducible. The risk of delivery before 37 weeks of gestation is increased significantly if the LUW thickness at 18-22 weeks is less than 4.5 mm.Trial Registration: The study protocol was approved by institutional ethical committee (COA No. Si 657/2016). [ABSTRACT FROM AUTHOR]