The aim of this study is to analyze the usefulness of the enoxaparin 4000 UI/day in the prevention of a composite maternal or perinatal morbidity (occurrence of one of the following events: maternal death, PE, fetal growth retardation, abruptio placenta, perinatal death) in women who previously had a severe preeclampsia at less than 34 weeks' gestation. To answer this question, the investigators propose to conduct a multicenter prospective randomized trial that will compare two groups in parallel: a group where women will have an association of enoxaparin 4000 U/day and aspirin 100 mg/day and another group where women would have only aspirin 100 mg/day. The number of patients needed is 255 (amendment n°2-approved 06/12/2011) .