Maymom 2nd Gen Thread Changer Bottle Adaptor is a pack of four bottle-size changers that is designed to avoid interfering with the duckbill, and easier screw on to both the Widemouth flanges and the standard-sized bottles. Allow Avent and Spectra S1 and Spectra S2 flanges t o use with standard-sized bottles like medela bottles, ameda bottles and original Dr. Brown Bottles (note will not fit wide mouth Dr. brown bottle) to replace the wide mouth spectra bottles. Connect Wide-neck flanges directly to Standard/Narrow-neck bottles Compatible with Spectra S1/S2 Flanges to use with Standard Size Botles like Medela/Original Dr Brown Bottles (Please refer to compatibility image) BPA free, dishwasher safe; FDA compliant for food contacts. Every batch is tested for safety. Includes 4x Maymom Thread Changer Bottle Adaptor ONLY.