How to Use the Willow Sizing Insert Set for a Comfortable Fit
By Willow: Wearable Breast Pump – Willow Pump | Published: 2026-10-13
Category: Guías prácticas
Learn how to use the Willow Sizing Insert Set to find your correct Willow flange size, avoid common sizing mistakes, and pump more comfortably.
Quick answer
Use the Willow Sizing Insert Set to test insert sizes against your nipple before you commit to a pump session. Measure your nipple base, try the insert that matches, and adjust down or up based on comfort and milk flow. A correct fit should feel snug without pinching, and you should see your nipple move freely in the tunnel.
Why the Willow Sizing Insert Set matters for wearable pump fit
A wearable pump only works well when the flange tunnel matches your nipple size. If the tunnel is too large, your nipple and areola get pulled in, which can cause pain and leave milk behind. If it is too small, the tunnel rubs and blocks flow. The Willow Sizing Insert Set lets you test sizes without buying multiple flanges, so you can dial in your willow flange size before a full pump session.
This matters because breast pump flange sizing is not one-size-fits-all, and your size can change over time. Starting with the insert set saves you from guessing and helps you avoid pumping without pain.
What you need before you start
Gather your Willow pump, the Willow Sizing Insert Set, a mirror, and a ruler or measuring tape. If you are using the Willow Go® Wearable Breast Pump, note that the kit comes with 21 mm and 24 mm flanges and 5 oz SplashGuard™ containers, so you can compare those against the insert sizes. The Willow Sync Wearable Breast Pump fits inside your bra and is hands-free, so you can test insert fit while wearing your normal pumping bra.
Do not test sizes right after a feed when breasts are soft, or when they are overly full and tender. A mid-cycle check gives you the most accurate sense of how the insert will feel during real use.
Step-by-step: How to use the Willow Sizing Insert Set
Follow these steps in order. Each step includes a checkpoint so you can catch a common mistake before it affects your comfort or output.
Your sizing steps and checkpoints
- Measure your nipple base: Use a ruler or measuring tape to measure the diameter of your nipple base in millimeters. Do this before pumping or after a short warm-up, not when you are engorged. Checkpoint: Write down the number. If you measure between two sizes, start with the smaller insert.
- Select the closest insert size: Match your measurement to the nearest Willow Sizing Insert Set size. If you are using the Willow Go pump, compare against the included 21 mm and 24 mm flanges to see how the insert changes the tunnel. Checkpoint: The insert should sit flush in the flange without gaps or wobble.
- Test the insert with a short pump session: Insert the sizing insert into your flange, center your nipple in the tunnel, and pump on a low suction setting for 2–3 minutes. Watch how your nipple moves. Checkpoint: Your nipple should move freely without rubbing the tunnel walls. Stop immediately if you feel pinching or pain.
- Adjust based on comfort and flow: If your nipple rubs or turns white, try the next size up. If your areola is pulled into the tunnel or milk output drops, try the next size down. Re-test after each change. Checkpoint: The correct size feels snug but not painful, and milk flows steadily without you pressing hard.
- Recheck your size as needed: Your breast and nipple size can change with milk supply, weight changes, or after weaning. Re-measure every few weeks or whenever pumping becomes uncomfortable. Checkpoint: If you feel pain or see reduced output, repeat the sizing steps before assuming the pump is the problem.
Product facts
- Willow Go® Wearable Breast Pump: Willow Go® Wearable Breast Pump kit comes with 21 mm and 24 mm flanges and 5 oz SplashGuard™ containers. You can use these flanges as a baseline while testing Willow Sizing Insert Set sizes, so you know which direction to adjust.
- Willow Sync Wearable Breast Pump: Willow Sync Wearable Breast Pump fits comfortably inside your bra for discreet, hands-free pumping. Because the pump sits in your bra, the insert must sit correctly against your nipple without shifting, making sizing checks especially important.
