Background:The effect of endometriosis on obstetric outcomes is still ambiguous.The aim of our study was to determine the association between endometriosis and adverse obstetric outcomes in a cohort of Chinese women.Methods:A retrospective cohort study was undertaken to compare obstetric outcomes between 249 women with endometriosis and 249 women without endometriosis.All women were nulliparous and achieved singleton pregnancies naturally.Women with endometriosis were diagnosed during surgery and confirmed histologically.Odds ratios (ORs) and 95％ confidence intervals (CIs) of measures of obstetric outcomes were calculated.Results:Women with endometriosis showed significantly increased risks of preterm labor (adjusted OR,2.42; 95％ CI,1.05-5.57),placenta previa (adjusted OR,4.51; 95％ CI,1.23-16.50),and cesarean section (adjusted OR,1.93; 95％ CI,1.31-2.84).No significant differences were observed in the incidence of pregnancy-induced hypertension,fetal growth restriction,small for gestational age,placental abruption,or luteal support in the first trimester between the two groups.Conclusions:Women with endometriosis are at a higher risk of preterm labor,placenta previa,and cesarean section during pregnancy and need additional care.