Abstract： Background Early onset severe preeclampsia is a specific type of severe preeclampsia,which causes high morbidity and mortality of both mothers and fetus.This study aimed to investigate the clinical definition,features,treatment,outcome and risk factors of early onset severe preaclampsia in Chinese women.üMethods Four hundred and thirteen women with severe preeclampsia from June 2006 to June 2009 were divided into three groups according to the gestational age at the onset of preeclampsia as follows:group A (less than 32 weeks,73 cases),group B (between 32 and 34 weeks,71 cases),and group C (greater than 34 weeks,269 cases).The demographic characteristics of the subjects,complications,delivery modes and outcome of pregnancy were analyzed retrospectively.üResults The systolic blood pressure at admission and the incidence of severe complications were significantly lower in group C than those in groups A and B,prolonged gestational weeks and days of hospitalization were significantly shorter in group C than those in groups A and B.Liver and kidney dysfunction,pleural and peritoneal effusion,placental abruption and postpartum hemorrhage were more likely to occur in group A compared with the other two groups.Twenty-four-hour urine protein levels at admission,intrauterine fetal death and days of hospitalization were risk factors that affected complications of severe preeclampsia.Gestational week at admission and delivery week were also risk factors that affected perinatal outcome.Conclusions Early onset severe preeclampsia should be defined as occurring before 34 weeks,and it is featured by more maternal complications and a worse perinatal prognosis compared with that defined as occurring after 34 weeks.Independent risk factors should be used to tailor the optimized individual treatment plan,to balance both maternal and neonatal safety.