This 6-piece Dr. Brown's baby bottle nipple bundle includes 2 packs of Level 3 medium-fast flow wide-neck nipples and 1 pack of Level 4 fast flow narrow neck nipples. The Level 3 nipples are designed for growing babies 6 months and up who are transitioning from a newborn flow rate. Their medium-fast flow rate provides a smooth, consistent flow so babies can feed at their own pace. The Level 4 fast flow nipples are ideal for babies 9 months and older who need a higher flow rate. Together, these BPA-free 100% silicone nipples complement a baby's growth stages. All Dr. Brown's nipples feature an internal vent system clinically proven to reduce colic by decreasing spit-up, burping and gas. Their breast-like shape supports proper latching for a natural bottle feeding experience. The anti-colic system also preserves nutrients and aids digestion. Parents can easily alternate between the Level 3 and Level 4 nipples as their baby's flow rate needs change. The Level 3 nipples fit Dr. Brown's wide-neck bottles, while the Level 4 nipples fit narrow bottles, providing parents flexible bottle options. With their innovative vent technology and soft silicone material, these nipples support healthy feeding and simplified bottle interchangeability.