ObjectiveTo determine whether obstetric outcomes differ between women with endometriosis and those without, where all women undergo first trimester screening for endometriosis DesignA prospective observational cohort study SettingThe Early Pregnancy Unit at University College London Hospital, United Kingdom SubjectsWomen with a live pregnancy progressing beyond 12 weeks’ gestation and concurrent endometriosis (n=110) or no endometriosis (n=393). ExposureAll women underwent a pelvic ultrasound examination in early pregnancy to examine for the presence of endometriosis and uterine abnormalities. Main outcome measuresThe primary outcome of interest was preterm birth, defined as delivery before 37 completed weeks’ gestation. Secondary outcomes included late miscarriage, antepartum haemorrhage, placental site disorders, gestational diabetes, hypertensive disorders of pregnancy, neonate small for gestational age, mode of delivery, intrapartum sepsis, postpartum haemorrhage and admission to the neonatal unit. ResultsWomen with a diagnosis of endometriosis did not have statistically significantly higher odds of preterm delivery (aOR 1.85 (95% CI 0.50-6.90)), but they did have higher odds of postpartum haemorrhage during Caesarean section (aOR 3.64 (95% CI 2.07-6.35);) and admission of their newborn baby to the neonatal unit (aOR 3.24 (95% CI 1.08-9.73);). Women with persistent or recurrent deep endometriosis after surgery, also had higher odds of placental site disorders (aOR 8.65 (95% CI 1.17-63.71);) and intrapartum sepsis (aOR 3.47 (95% CI 1.02-11.75);). ConclusionWe observed that women with endometriosis do not have higher odds of preterm delivery, irrespective of their disease subtype. However, they do have higher odds of postpartum haemorrhage during Caearean section and newborn admission to the neonatal unit.