Switching from bottle back to breast has never been easier. There's a reason, calma looks different than other nipples on the market. The unique design is research based and proven to allow your baby to use the same breastfeeding behavior learned at the breast when feeding with calma. This design ensures that calma does not create nipple confusion, therefore does not interfere with breastfeeding. Yes, there is only one calma size. Calma is designed to adjust to your baby's needs as your baby grows, so you do not need to purchase different nipples for different stages. The length, shape and flow of the nipple was determined by research and is suitable for the full-term babies. Don't be fooled by calma's opening. The opening of calma is not indicative of the size of the milk flow. The milk actually flows through a very small hole inside the calma. The flow speed is dictated by your baby: slow flow for the tiniest of babies to faster flow as your baby grows and gains sucking power, just as they would when breastfeeding. Calma helps avoid gasiness as it is vented through our unique air-control system. Calma requires your baby to use the same facial muscles s/he would use during breastfeeding in order to create suction which fosters healthy oral development. Plus, calma doesn't' drip so you don't have to worry about cleaning up spills, allowing you to spend more time on what matters. Calma is BPA-free, and made without DEHP and other phthalates.