The investigators hypothesized that the "Pecs blocks 1 and 2" will provide a decrease of the maximal pain of the first 6 post-operative hours after prosthetic breast expansion. The pain will be assessed according to a numerical scale. This prospective, randomized, double blind study will include 92 patients successively during two years, randomized into two arms : Pecs group and Control group, which will respectively have the block, and no block. The rest of the anesthesic and surgical care will be identical.