Compatibility: These bottles are compatible with all Medela breast pumps and Calma, allowing you to pump, store, and feed using the same container. This ensures that not a single drop of precious breast milk is wasted. Storage: Perfect for storing breast milk, these bottles can safely go in the fridge or freezer, preserving the nutrients of the milk. Leak-Proof Design: The bottles feature screw-on lids to prevent leaks when stored in the fridge or freezer, or when transported during travel. This provides peace of mind for breastfeeding mothers on the go. Measurement Markings: Clear ounce and millimeter markers on the bottles make it easy to measure the exact amount of milk you have expressed, allowing for accurate feeding and monitoring of milk supply. Complement to Breastfeeding: The Medela breast milk bottles are designed to complement your breastfeeding experience, providing a convenient and reliable storage solution for expressed breast milk. Overall, the Online Only Medela Breastmilk Bottles 150ml 3 Pack offers practicality and ease of use for breastfeeding mothers, ensuring that expressed breast milk is safely stored and readily available whenever needed.