Exclusively for Dr. Brown's Natural Flow baby bottles Made of super-soft, high-grade silicone, each nipple level provides a different flow rate. All of these bottle nipples vary in size and shape. From the Preemie Flow nipple, which limits the flow to tiny mouths to the Y-Cut nipple which literally utilizes a "Y"-shaped opening to allow for greater milk flow, youll find the bottle nipple thats the perfect fit for your babys feeding needs. One size does not necessarily fit all. Keep in mind that the Bottle Nipple Chart serves as a general guideline for choosing a bottle nipple but is by no means an end-all solution. It is important to carefully watch the feeding habits of your little one. For instance, is your three-month-old not seeming to get enough from the bottle at feeding time? Moving her up from a Level One to a Level Two bottle nipple may be in order. Adapt your bottle nipple selection to match your babys feeding style, regardless of age. Size Chart: Preemie Flow - For Preemie Babies Level One - For Newborns 0+ months Level Two - For Babies 3+ months Level Three - For Babies 6+ months Level Four - For Babies 9+ months Y-Cut - For Babies 9+ months. - Level 2 (3m+)