The purpose of this study is to determine whether, in a high risk population (placenta praevia and previous caesarean or prenatal suspicion of morbidly adherent placenta (MAP)), the concentration of cell-free fetal DNA circulating in the maternal plasma is significantly increased in the subgroup of morbidly adherent placenta (MAP) cases , in order to determine if the dosage of cell-free fetal DNA circulating in the maternal plasma may be a useful biological tool to detect MAP, alone or in addition to the imagery findings (ultrasonography and RMI).