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 trying to replace the shields that came with your pump, this is probably the size you need. You will also need
Personal Fit Connectors to use this product. 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. If you need more help choosing a shield
read this article.
For hygienic reasons, breast pump parts, storage, and bottles cannot be returned once opened.