PersonalFit Flex Breast Shields, 36mm.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 better pumping results. Medela PersonalFit Breast Shields are made without BPA. Authentic Medela Spare Parts are engineered, tested and approved to work specifically with your Medela breast pump. Non-Medela spare parts can very significantly in design, materials and workmanship; all of which may affect the performance of your Medela breast pump.