This BPA-Free Polypropylene Bottle 4 oz. from Dr. Brown's reduces colic using a patented vent system. This patented Natural Flow internal vent system is the only vent that eliminates the vacuum and the air bubbles found in most baby bottles. It allows air to enter through the nipple collar without being incorporated into the breast milk or formula. The air is then channeled through the internal vent system to the area above the breast milk or formula, allowing babies to feed more comfortably and without fighting the negative effects of a vacuum or the discomfort of ingesting air bubbles. BPA-, PVC-, lead- and phthalate-free. Includes one 4 oz. standard polypropylene baby bottle, one Level-One silicone nipple, one two-piece patented internal vent, one cleaning brush and instructions.