Ultra soft and precision cut, Dr Browns narrow neck level 4 teats, 9+ months, optimises performance to ensure maximum flow of liquid. It is made with a high grade, food safe silicone a patented twopiece internal vent system. This vent system, fully expresses the bottle for vacuumfree feeding, which is comparable to breast feeding. As your baby feeds, air is channelled from the teat collar through the vent system, bypassing the breast milk or formula, to the back of the bottle. In this manner, oxidation is kept to a minimum as the air never mixes with the breast milk or formula. Furthermore, this technology ensures optimal comfort for your baby, as it allows them to nurse without the discomfort of ingesting air bubbles; thereby, reducing feeding problems like colic, burping & wind. The breast milk or formula flows freely, without teat collapse.