Objective To investigate the association of endometrial thickness(EMT)with obstetric and neonatal outcomes of monoparous pregnancy in fresh cleavage embryos transfer.Methods A total of 1 845 patients of monoparous pregnancy after fresh cleavage embryos transfer cycles from Jan 2016 to Mar 2022 at Shanghai First Maternity and Infant Hospital,Tongji Universtiy were analyzed retrospectively.Patients were categorized into three groups by EMT on transferation day:≤8 mm(group A),8-14 mm(group B)and≥14 mm(group C).The primary outcomes were preterm birth(PTB),birth weight and birth weight z-score,small-for-gestation age,large-for-gestation age,very low birth weight,low birth weight and macrosomia.The second outcomes were pregnancy and perinatal complications.The relationship between EMT and adverse neonatal outcomes was estimated by Logistic regression analysis.Results The rate of ectopic pregnancy was increased significantly in group A.No significant differences were found among the three groups in gestation age,birth weight,birth weight z-score,PTB,small for gestation age,large for gestation age,low birth weight,very low birth weight and macrosomia.Compared with group B,the odds of adverse neonatal outcomes did not show significant differences before and after adjustment in both group A and group C by Logistic regression analysis.Conclusion Thinner EMT in fresh cleavage embryos transfer is associated with higher rate of ectopic pregnancy,while it is not independently associated with adverse perinatal outcomes.