It is well known that using the correct breastshield size is important for your pumping comfort and optimal milk supply. It is also known that one-piece design offers easy to clean, fewer pieces to keep, and no seams to lose suction power. <p>Maymom is the first company world-wide to offer 15, 17, 19, 21, 25, 26, 27, 29, 30, 34 and 36 mm internal diameter one-piece design breastshields to fit Medela users personal pumping needs. The valve/membrane in this set are interchangeable with your existing Medela parts. Maymom has 4 years experience in making breast pump parts. 100% compatibility is guaranteed or your money back! <p><b>Disclaimer:</b> Medela and pump names (Pump in style, Swing, Freestyle etc) are registered trademarks of Medela Holding AG. Maymom is not affiliated with Medela nor does Medela endorse the product.</p>