In the menstrual cycle, a slight elevation of plasma FSH levels, the inter-cycle FSH signal, initiates the 1st phase of follicular recruitment. In ovarian stimulation for IVF, exogenous FSH aims at increasing the amplitude of this signal while maintaining FSH levels elevated, in order to fool the natural mechanisms of single follicular dominance. Synchronization consists in maintaining plasma FSH levels low so that all recruitable follicles remain at a resting state, prior to inducing ovarian stimulation. Practically speaking, synchronization of ovarian follicles allows to optimize clinical efforts, reduces the risk of cyst formation and ultimately, helps improving IVF outcome.