Our 3-pack of Flange Inserts (17/19/21mm) is perfect for finding the perfect fit for your nipples and is compatible with the Baby Bliss Wearable Pump. The right size of flange insert is key for successful and comfortable pumping. Some moms may experience nipple swelling while expressing milk, so it's important to monitor your nipples during pumping. For optimal use, we recommend replacing your flange insert every 1-2 months. Our product is crafted from BPA-free transparent silicone, so you can trust that you're using a safe, high-quality product.