Determine the diameter of your nippleThe standard breast shield that comes with the Mon Lait Pump is 24mm, however you may need a smaller or larger flange based on your measurement.
Nipple to Breast Shield Size Guide:
Nipple size up to 15mm - 17mm Flange Inserts
Nipple size up to 17mm - 21mm Flange Inserts
Nipple size up to 20mm - 24mm Breast Shield (Comes with the Pump)
Nipple size up to 25mm - 27mm Breast Shield
(See image below)