Created by a physician, dr. Brown's natural flow baby bottles feature a patented internal vent system that helps reduce colic, spit-up, burping and gas. The positive pressure design allows babies to feed comfortably, because the vent system eliminates the air bubbles and vacuum. All parts are dishwasher safe. Two replacement silicone nipples, exclusively for dr. Brown's natural flow standard baby bottles. Level four nipples are ideal for babies 9+ months. .