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, you'll find the bottle nipple that's the perfect fit for your baby's 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 baby's feeding style, regardless of age.