The purpose of this study is to evaluate the use of functional MRI in pregnant women as a non-invasive diagnostic tool to detect placental insufficiency and differentiate healthy fetuses from the intra-uterine growth restricted ones. Functional MRI in pregnant women can detect a variation of the MRI signal (called BOLD effect) from the placenta and the fetus when the mother is breathing pure oxygen. This study aims hence to demonstrate the difference in the BOLD effect between normal feto-placental units and growth restricted ones.