Available sizes: 15mm, 16mm, 17mm, 18mm, 19mm, 20mm, 21mm
Properly sized flanges are optimal for comfort and effective breast emptying when pumping.  Inserts fit into your 24-25mm flanges to decrease the tunnel size for smaller maternal nipple diameters. 2 flanges come with each size purchased. 
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Mostly compatible with the following pump equipment:
24mm Spectra S1, S2/Medela PersonalFit, Flex Flange
24mm Elvie, Stride/Willow/Momcozy S9, S10, S12 Cup
24mm TSRETE/Bellababy/Rainyb/Aidmom/HAUTURE Cup
24mm Motif Luna/NCVI/Bellababy Flange
25mm Freemie/Ameda Mya/Ameda Mya Pro Flange
Purchase of flange inserts are non-refundable.