A vent diaphragm for a baby bottle, configured to interface with the bottom of the bottle which is formed with one or more bores for the passage of air from an outside environment to the interior of the bottle, said diaphragm comprising: - a central flexible portion which is elastically deformable to reversibly move between a rest configuration, when the pressure inside the bottle is equal to or higher than the pressure of the outside environment, and a vented configuration when the pressure in the bottle is lower than the pressure of the outside environment, - a first ring portion surrounding said central flexible portion, said first ring portion being concentric and connected with the central flexible portion; - a second ring portion surrounding said first ring portion, said second ring portion being concentric and connected with the first ring portion; said central flexible portion being raised with respect to said second ring portion toward the interior of the bottle, said first ring portion connecting said central flexible portion and said second ring portion; characterized in that said central flexible portion comprises: - a central dome-shaped zone; - an annular depression externally surrounding said central dome-shaped zone, said annular depression being concentric and connected with said central dome-shaped zone; - an annular rib surrounding said annular depression, said annular rib being concentric and internally connected with said annular depression and externally connected with said first ring portion; and wherein: - said annular rib comprises a plurality of passages configured to move between: - an open position to allow passage of air from said outside environment to the interior of the bottle when the central flexible portion is in the vented configuration; and - a fluid-tight closed position when the central flexible portion is in the rest configuration. - a central flexible portion which is elastically deformable to reversibly move between a rest configuration, when the pressure inside the bottle is equal to or higher than the pressure of the outside environment, and a vented configuration when the pressure in the bottle is lower than the pressure of the outside environment, - a first ring portion surrounding said central flexible portion, said first ring portion being concentric and connected with the central flexible portion; - a second ring portion surrounding said first ring portion, said second ring portion being concentric and connected with the first ring portion; said central flexible portion being raised with respect to said second ring portion toward the interior of the bottle, said first ring portion connecting said central flexible portion and said second ring portion; - a central dome-shaped zone; - an annular depression externally surrounding said central dome-shaped zone, said annular depression being concentric and connected with said central dome-shaped zone; - an annular rib surrounding said annular depression, said annular rib being concentric and internally connected with said annular depression and externally connected with said first ring portion; - said annular rib comprises a plurality of passages configured to move between: - an open position to allow passage of air from said outside environment to the interior of the bottle when the central flexible portion is in the vented configuration; and - a fluid-tight closed position when the central flexible portion is in the rest configuration.