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. Medela offers most breastshields only in the two-pieces form. Maymom is the first company world-wide to offer 19, 21, 25, 26, 27, 29 and 30 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!