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 & vacuum. Dishwasher safe. Polypropelene set includes 6-8oz bottles;4-4oz bottles;10-Level 1 nipples;4-Level 2 nipples;4-Level 3 nipples; 4-Level 4 nipples;10-2pc patented internal vents;6 Storage Caps;5 Cleaning Brushes. BPA FREE