The Truth About Pump Flange Sizing

If you are pumping and things feel uncomfortable or just not quite right, this one is for you.

For a long time, many of us were told to use bigger pump flanges to “protect the nipple” and prevent rubbing. It sounded logical. More room means less friction, right? I remember using a size 28mm when pumping for my son when now I know I should have used a size 13mm!

But here is what we understand now:

When tissue is placed in a negative pressure vacuum (like a breast pump tunnel) it naturally swells. So if a large amount of nipple and areola are pulled too far into the flange, what often happens?

Swelling
Tenderness or soreness
That puffy, stuck feeling
And sometimes lower milk output

This swelling is your body responding to too much space and too much pull. It is not something you are doing wrong.

Here is the key. Comfort and milk flow improve when the vacuum is concentrated on the nipple itself, not the entire areola.

The old way versus the updated approach

Old thinking: Give the nipple lots of space so it does not touch the sides.

Updated understanding: A snugger fit, where the nipple gently glides in the tunnel, often feels better and works better.

Why this matters:

A focused vacuum can support oxytocin release
Better oxytocin supports a stronger milk ejection reflex (aka, your "letdown")
Less tissue pulled inside means less swelling and discomfort

Parents frequently share that once they try smaller sizes with guidance it feels entirely different in the best way.

What the latest research tells us

A 2024 pilot study compared traditional flange sizing (larger sizes, space around the nipple) with smaller, measurement-based sizing. Results showed:

More comfort
More milk output
Most commonly used sizes were 15mm and 17mm!

These are much smaller than the sizes included with most pump kits.

"Participants reported significantly higher comfort and more efficient milk removal using smaller flanges sized by nipple tip measurement and real-time trialing."

This study also emphasized that flange sizing works best when you try sizes with the pump running, since nipples can change shape and size under vacuum. And yes, it is normal for one side to need a different size than the other. Remember, your nipples are sisters not twins!

What the Flange FITS Guide teaches

The Flange FITS Guide by Jeanette Mesite Frem, IBCLC reinforces that:

Most people do best starting near their nipple measurement
Try 2 to 3 sizes while pumping
Look for comfort, a gentle glide, and visible sprays of milk
Larger flanges can increase swelling and reduce efficiency

"The nipple should glide comfortably in the tunnel without excessive swelling or pulling of the areola into the flange."

This guide also supports experimenting with flange materials, shapes, and tempo settings, because everyone responds differently.

So how do I know what size I need?

There is no one formula or universal chart. Flange fitting is individualized.

A proper fitting session typically includes:

Measuring nipple tip diameter
Trying nearby sizes with the pump running
Watching nipple movement and milk flow
Checking comfort throughout the session
Adjusting pump settings to match your body

A good rule: pumping should feel comfortable, light, and effective. Not pinchy. Not pulling in your whole breast. Not something you have to tolerate.

Why this matters

The wrong flange size can impact:

Comfort and nipple health
Milk supply
Pumping time and efficiency
Risk of swelling or nipple trauma
Your confidence and mental load

Many people do not need the 21 mm and larger flanges that come in the box. Research in this area is expanding rapidly, and as new sizes and materials are released, we now have more tools than ever to help you find the right fit.

You deserve comfortable and effective pumping

You should not have to suffer through pumping or guess your size.

If pumping hurts, feels inefficient, or you are not sure your flange size is right, we can help.

During a sizing session, we try multiple flanges with the pump actually on to find your comfort and output sweet spot.

Book a session & let's get your pump flange fit just right and make sure you feel comfortable with your pump (available in person or virtually)

You deserve tools and guidance that work for your body.