This Spectra Breast Shield Flange is designed for use with the Spectra S1 Plus, Spectra S2 Plus, Spectra Dew 350, and Spectra 9 Plus breast pumps and is compatible with Spectra bottles with a wide neck ( if you are double pumping, you will need two flanges). It might be difficult to know which size flange you need because your flange size has nothing to do with your breast size it has to do with the size of your nipples. Some moms will experience nipple swelling while pumping, so it's important to observe how your nipples look during a pumping session. The size of your flange is important for comfort and efficient pumping; while pumping, your nipple should not rub or feel squished inside the flange tunnel, and all of your nipple should fit into the flange tunnel and move in and out freely. If you feel any nipple discomfort, pain, or blanching (turning white) while pumping, you may need a larger flange. Some moms may need a different sized flange for each breast, and this may change over time.