Why choose Momcozy Wearable Breast Pump Flange Inserts? Momcozy Flange Inserts 15/17/19/21mm are designed for use with moms, Compatible with 24mm Flange/Shield of Most Breast Pumps It can sometimes be difficult to know what size flange insert you are. Your flange size has nothing to do with your breast size, you can use the picture guide on the link to measure the best flange size for you, which can help you improve the efficiency and comfort of breastfeeding. Some mothers experience nipple swelling while expressing, so it is important to observe the appearance of the nipple during pumping. The size of the flange is significant for comfortable and efficient pumping. Your nipples should not rub or feel squeezed in the flange channel when expressing. All your nipples should fit in the flange channel and be free to go in and out. If you experience any discomfort while pumping, you may not be using the right size flange and need to replace it with a new size flange. Some moms may need a different flange size on each breast, which can change over time. Therefore, it is recommended that you make a selection after accurate measurements to confirm the correct flange size. If there is any problem during use, please contact us in time, and we will provide you with different liberation solutions as soon as possible to help you solve the problem, thank you. Attention: Flange Inserts 13/15/17/19/21mm are only suitable for 24mm Shields/Flanges, not for 27mm Shields/Flanges or another size.