For maximum comfort and pumping efficiency, Medela offers five breast shield sizes. This guide is a starting point to help determine your optimal size.
Reasons to try a new size
•    Does your nipple rub the tunnel sides to the point of causing discomfort?
•    Do you see excessive areola being pulled into the tunnel?
•    Do you see any redness?
•    Is your nipple or areola turning white?
•    Do you feel unexpressed milk after breast milk pumping?

If you answered "YES" to any of these questions, consider trying a new size by following the measuring instructions below.
If you are still unsure that you selected the right breast shield size, contact a lactation consultant or breastfeeding specialist.
 

Understanding Medela’s PersonalFit sizing
Step 1
Using a ruler or measuring tape measure the diameter of your nipple at base (across middle) in millimeters (mm). Do not include areola.
Step 2
Based on your breast measurement, determine your PersonalFit nipple shield size.
Example: If your nipple size measures 17 mm in diameter, the recommended PersonalFit breast shield size is 21 mm.
 
Test your breast shield size
•    Start with the 24 mm that came with your pump, or the size determined by measuring.
•    Center the nipple and gently hold the breast shield against your breast.
•    Refer to the images below while pumping in the expression phase (after the stimulation phase).