3. Statistical analysis: Women with previous severe preeclampsia at less than 34 weeks' gestation have a 40% risk of occurrence of a composite morbidity (primary outcome defined above) at the next pregnancy. A 33% decrease of the composite morbidity defined above in women treated with LMWH enoxaparin is considered to be efficient. With an alpha risk of .05 and a beta risk of .20, the number of patients needed is 255 (amendment n°2-approved 06/12/2011) . This trial will be analyzed as an "Intention to treat study". No interim analysis for the primary outcome will be performed. Results will be stratified by center.