First, what’s a flange? 

A flange is a cone-shaped funnel, typically ending in a tunnel, that seals over your nipple and breast and creates a vacuum to transmit milk. Sizes are based on the diameter (or width) of your nipple (not your areola, which is a common misconception!). 

For moms who measure smaller than 21mm, breast pump sizing inserts, made of flexible silicone, optimize comfort and output. (It’s a way to get a more custom-feeling fit!) Willow inserts are designed to fit into a 24mm flange and are available in sizes 13mm, 15mm, 17mm, 19mm and 21mm. 

Based on years of sizing data (more than 300k women have pumped with Willow!), we know that most moms find the best fit using a 17mm or 19mm insert.

Why does sizing matter?

A poorly-fitting flange can impact stimulation, which can then affect the hormonal rush of oxytocin that triggers letdown and milk production, and decrease output in turn. 

If your flange is too large, you’ll have less pressure on the alveoli and less stimulation overall, which can result in insufficient emptying. (And when you leave milk behind, you may decrease your supply.) If your flange is too small, your nipple may rub against the tunnel, causing discomfort, pain, and compressed milk ducts that restrict emptying.

What else impacts fit?

Aside from nipple size, finding the right fit also depends on your skin elasticity and breast tissue sensitivity. Once you know what flange or insert might be right for you based on your measurements, don’t be afraid of experimenting. You may find that using a flange or insert a size above or below your measured size is more comfortable and effective.

When should I measure? 

First and foremost, you should measure anytime you feel any discomfort when pumping, or experience a sudden change in output. Remember to always measure both nipples — no two breasts are the same (even if they’re both yours!), and they may change in size throughout your breastfeeding journey. 

Can I measure my nipples when I’m still pregnant? What about when my milk comes in?

Your nipples will undergo a series of changes in both shape and size as you shift from pregnancy to establishing breastfeeding to reaching a steady state. It’s normal to need a different flange size a few times before you finish your pumping journey, so be prepared to measure a few times—and possibly swap out your flanges or inserts each time. 

If you’re still pregnant and want to get an idea of your flange size, the ideal time to do so is before your sixth month of pregnancy. After that, they may actually be larger than they’ll be during most of your pumping and breastfeeding journey.  

If you’ve purchased your flanges during pregnancy, you’ll want to measure them again once you start pumping — ideally before you pump or at least an hour after you’ve finished. If you’re sized in the hospital, this is the stage when your nipples are likely at their largest, so you should resize again in a few weeks — ideally around six to eight weeks postpartum.

In summary? You can measure your nipples when you’re pregnant — just remember to do it before the six-month mark. If you’re sized at the hospital, take your measurements again after 6-8 weeks (you should have reached a “steady state” at this point), as it’s possible your nipples may be smaller than they were right after giving birth. After that, measure any time you notice pain, discomfort, or a change in supply.

How do I measure? 

The good news is that it’s easy to measure yourself at home. We recommend sizing with Willow's cut-out nipple sizing tool, or printing it out if you don't have a copy handy. Alternatives include using a cloth measuring tape or ruler (physical or digital works!).

Once I have my measurements, which size do I choose?

When it comes to wearables, you’ll need a fit that’s more snug than what you might be used to with a traditional pump. Most moms find that adding 1-3mm to their measured nipple size (or 0-2mm if you’re pumping with the Willow 360) results in optimal output and comfort. After measuring the width of your nipple, use this handy sizing chart to find your size.

How do I know I have the right fit?

Center your nipple in the flange, and start pumping. You should feel pulling, not pinching. If your nipple rubs against the side, it’s likely too small. If you don’t feel suction, it’s likely too big.