A vent diaphragm for a baby bottle, adapted 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, said central flexible portion comprising at least one passage 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 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; - an annular stiffening member adapted to at least partially stiffen the first ring portion, characterized in that the assembly of the first ring portion with the central flexible portion has a dome shape. - 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, said central flexible portion comprising at least one passage 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; - 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 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; - an annular stiffening member adapted to at least partially stiffen the first ring portion,