Breast pump flanges are not one-size-fits-all. Most pumps include a standard 24-25mm flange; this indicates that the diameter of the tunnel, the area where you place your nipple, is 24-25mm across. That’s not the right size for all users though; there are a range of sizes available, as small as 13mm and as large as 36mm.
To fit your flange; you need to position your breast within the flange and turn on your pump. If both your nipple and your entire areola are being pulled with the suction of the pump; that means the flange is too large for you. Conversely, if only a small portion of your nipple is being pulled in and is rubbing against the sides of the tunnel, you probably need a larger flange.
So what does a good fit look like? Your breast should be firmly against the tunnel and the nipple should move freely within the tunnel of the flange. Upon completion of the pumping session, take note of the appearance of the breast; if there is a great deal of swelling; trying a different size is recommended.
Sometimes, it’s evident that no standard flange is working well; this indicates that you may need a flange that offers a different downward slope than the typical flanges allow. These are available and can help with fitting your unique breast shape to give you a more comfortable pumping experience. When choosing a flange size, keep in mind that the size of the flange is determined by the size of your nipple and not by the size of the breast itself. You want your nipple to be pulled comfortably into the pump with very little areola being pulled in as well. The nipple should move freely within the tunnel of the flange without any rubbing against the sides. Also stay aware of swelling; lots of swelling can indicate a bad fit. A well-fitting flange is not only comfortable but also helps yield the largest volume of milk.