Medela PersonalFit Breastshields are designed to accommodate different nipple sizes. They are available in five different sizes, allowing
mothers to choose exactly the right breastshield size for them to
ensure maximum comfort and output whilst expressing milk. Pumping
should be comfortable!
The 21mm shield is the smallest size made by Medela. If your nipple is not being pulled down into the lower part of the 24mm shield, you may want to try this smaller shield.
The size shield that Medela provides with new Medela breastpumps is the medium (24 mm). Some pumps also come with the large (28mm) size. You can verify size size by of a shield by looking at the outside of the flange and searching for small, raised text that will read the size. Based on that size, you can change the size you are using to meet your current needs.
Size up if you have large nipples and they rub against the inside of
the shield when you pump causing some discomfort (and maybe even
incomplete emptying of the breast) and/or if pumping is actually painful to the nipple.
Size Down if the nipple is not pulled down into the breastshield when pumping.
a breastshield that is the wrong size for you can lead to decreased
milk production. If you are not sure about breastshield size, you may want to consult a Lactation Consultant.