The aim of this work is to develop a biomechanical Finite Element (FE) breast model in order to analyze different breast compression strategies and their impact on image quality. Large breast deformations will be simulated using this FE model. A particular attention will be granted to the computation of the initial stress in the model due to gravity and to boundary conditions imposed by the thorax anatomy. Finally, the model will be validated by comparing the estimated breast deformations under gravity load with the experimental ones measured in three body positions: prone, supine and oblique supine.