One of the most common (and most commonly missed) reasons why mothers experience expressing difficulties is an ill-fitting breast pump flange. With breast pump flange size most certainly doesn’t fit all. The flange is the part of the breast pump that fits onto your breast with the tunnel into which your nipple gets pulled. Flanges are available with different diameter tunnels. When choosing a breast pump it’s important to check if the manufacturing company has different flange sizes available. Most ‘bottle-brand’ pumps that you buy off the shelf do not. In fact, these often do not even state their flange sizes on the box, so you may want to test one from a friend before making the choice to purchase.
Manufacturing companies typically have flange sizes available between 21mm and 30 mm. There are bigger flange sizes made for women with larger nipples, but these would need to be specially ordered and may not fit all pumps. It is generally recommended that you choose a flange size slightly larger (about 2- 4 mm) than your nipple diameter. You will obviously need to work with what is available. If you are unsure you can start expressing with the standard size that comes with the breast pump, and then up or downsize if needed.
With a ruler or measuring tape, measure the diameter of your nipple (do not include the areola). Your nipple size changes in pregnancy so you may want to wait until you actually start expressing before doing this. In some breastfeeding women, it may also change over time, so if you start experiencing problems you should recheck the fit. This is only likely to be an issue if your fit was borderline in the first place. Occasionally you may even need different flange sizes for your two nipples.