The Medela Calma is a unique alternative for nursing mothers wishing to also feed their babies with a bottle. Calma enables babies to maintain their natural feeding behavior learned on the breast. Supports an easy transition from the breast to the bottle and back. One size is sufficient for the entire breastfeeding period, just as in nature. Calma's unique design allows babies to suck, swallow and breathe, as learned on the breast. Whether you breastfeed or use Calma, the baby has to create a vacuum for breastmilk to flow. As soon as the baby pauses, the flow is stopped. Experts recommend introducing a bottle only after breastfeeding is established. Includes 1 all-stages nipple, 1 5 ounce BPA-free bottle, and 2 lids. Calma nipple is compatible with all Medela bottles as well as other standard-sized bottles. BPA and PVC free.