Using the correct size Breast Shield with your pump can help you to pump more effectively--you can get more milk, more comfortably. Medium is the default Medela size, which comes with all their pumps, and is the right size for most moms.  If you are pumping with size medium shields and experiencing rubbing or soreness along the sides of your nipples you may want to take a look at the Large Breast Shields or the XL Breast Shields.  You will also need Personal Fit Connectors to use any of the Personal Fit Shields.   If you need more help choosing a shield read this article.

Works with all Medela pumps (with personal fit connectors).  BPA Free

For hygienic reasons, breast pump parts, storage, and bottles cannot be returned once opened.