Dr. Brown's BPA-Free Natural Flow 4 oz. Baby Bottle helps maintain vitamins C, A and E. The patented Natural Flow internal vent system is the only vent that eliminates the vacuum and the air bubbles found in most baby bottles, allowing air to enter through the nipple collar without being incorporated into the breastmilk or formula. Babies feed more comfortably because the vent allows them to nurse without fighting the negative effects of a vacuum or the discomfort of ingesting air bubbles. This helps to reduce colic, spit-up, burping and gas. Includes two 4 oz. wide-neck bottles, two level-one silicone nipples, two two-piece patented internal vents, one cleaning brush and instructions.