One size does not always fit all, so Medela developed PersonalFit™ Breast Shields in a range of sizes to meet every mom’s needs. Lactation professionals agree that proper Breast Shield fit is essential for not only comfort, but also for effective milk expression. Authentic Medela spare parts are engineered, tested and approved to work specifically with your Medela breast pump. Non Medela spare parts can vary significantly in design, materials, and workmanship. All of which may affect the performance of your Medela breast pump. All Medela breast milk bottles and products that come in contact with breast milk are and always have been made without BPA. 21 mm.