Comes with an internal vent which reduces air bubbles and in-ear fluid. Now Moms can say "goodbye" to colic, spit-up, burping, gas, and everything in-between. The ergonomic bottles have an easy-to-read formula level gauge, and nipples are resistant to collapse. Connects directly to breast pumps. No expensive liners to buy! Easy to clean and dishwasher safe. *** All Stanford products are unable to be shipped via 2nd Day and Overnight Shipping methods. ***