The Dr. Brown"s Natural Flow baby bottle is the only baby bottle that features a patented internal vent system. This vent eliminates the air bubbles and vacuum effect to allow positive-pressure feeding with no air - similar to breast-feeding. Designed to: Reduce colic, spit-up, burping, and gas Prevent air bubbles from forming in milk or formula Help reduce fluid in the ear Prevent nipple collapse Features: Made from BPA free, polypropylene plastic Easy to clean and top-rack dishwasher safe Connects directly to most breast pumps Includes 8-oz Standard bottle, one 4-oz bottle, plus two extra Level-Two nipples