Pump Flange Fit Basics: Why the Old Rules Don’t Always Work Anymore

Educational only. Always consult your IBCLC for individualized care.

If you’ve ever tried pumping and wondered why it felt uncomfortable, noisy, ineffective, or just plain confusing… you’re not alone. Pumping is supposed to mimic what a baby does at the breast, but for years, many parents were given sizing guidelines that didn’t actually match how real nipples behave under suction.

For a long time, the standard advice sounded something like this:

“Measure your nipple and choose a flange a few millimeters bigger.”Or: “Most people need a 24 or 27mm flange.”

And while that method works for some people, we now know it doesn’t work for many. Thanks to newer research and clinician-led observations — especially from IBCLCs like Jeanette Mesite Frem of Babies in Common — we’re starting to understand that the old sizing rules were based more on pump manufacturer defaults than on human anatomy.

Let’s talk about what’s changed, and how to actually find a flange size that works for your body.

The Old Thought: Measure, Add 2–3 mm, Pick a Size From the Box

Traditional sizing guidance often led parents to flanges that were too big, because pumps were historically manufactured with only a few sizes: 24mm and 27mm.

Parents were told:

• Measure the nipple right at the base

• Add 2–3 mm

• That’s your flange size

The problem?Your nipple does not stay the same diameter while pumping. Suction pulls it forward, tissue stretches under vacuum, and the actual movement inside the flange tunnel matters more than a static measurement with a ruler.¹

Thousands of parents — especially first-time pumpers — ended up in flanges that were too large, leading to:

• Friction and discomfort

• Areola being drawn too deeply into the tunnel

• Swelling and edema

• Decreased milk output

• The belief that “pumping just hurts”

When in reality… it was the wrong size.

The New Thought: Smaller Flanges Often Work Better (and the Data Supports It)

In recent years, IBCLCs, lactation researchers, and pumping parents have noticed a clear trend:

👉 Most parents actually pump more comfortably and effectively with smaller flange sizes — often 13–19mm.

Jeanette Mesite Frem’s work, along with discussions across the lactation community, highlights that:

• Nipple measurement alone does not predict ideal flange size

• Nipples stretch differently under vacuum

• Pump suction affects tissue movement

• Effective pumping depends on comfort, mobility, and milk flow — not the number on the flange

Her observations and shared data sets show that a huge percentage of pumping parents never needed a 24 or 27mm flange in the first place. Many achieve better output and far more comfort using smaller inserts or narrower tunnel options.

In other words:**Flange fit should be based on how it feels and how it functions — not just on measuring.**²

So What Does Good Flange Fit Look Like?

Here’s what parents should look for:

✔ Your nipple moves freely in the tunnel

It should glide in and out without rubbing the sides.

✔ Minimal or no areola is drawn into the tunnel

Some movement is normal, but deep pulling or swelling is a sign the flange is too big.

✔ The sensation should feel like tugging — not pinching

Pain almost always means something is off: size, settings, suction, or positioning.

✔ Your nipple stays centered

If it repeatedly pulls to one side, the size or alignment might need adjusting.

✔ You see milk flow without needing high suction

Suction should feel comfortable and effective — not extreme.

✔ Your output is stable or improves

A good fit supports milk removal. A poor fit can restrict it.

Why Flange Fit Matters So Much

Poor flange fit can lead to:

• Pain

• Nipple swelling and trauma

• Slowed milk flow

• Reduced supply over time

• Plugged ducts from incomplete drainage

  
  

A proper flange fit can dramatically improve:

• Comfort

• Output

• Efficiency

• Breast health

• Long-term supply stability³

  
  

Many parents are shocked by the difference a correctly fitted flange makes — not because their body changed, but because the information finally did.

How to Find Your Best Size

Here’s the updated, evidence-informed approach IBCLCs now recommend:

1. Start smaller than you think.Your “true fit” may be several millimeters below your measured diameter.

2. Try sizes between 13–19mm if available.Most parents fall in this range, despite decades of misinformation.

3. Judge by comfort and milk flow — not by the number.

4. Reassess during postpartum changes.Swelling, hormonal shifts, and pump technique can affect optimal size.

5. Work with an IBCLC experienced in pump flange fitting if you’re unsure.

   
   

The Bottom Line

For years, parents were given limited flange sizes and one-size-fits-all instructions. Now we know better.

Breast and chestfeeding bodies deserve tools that fit them, not the other way around. A good flange fit is not about matching a millimeter measurement — it's about supporting comfort, milk flow, and long-term supply.

If pumping feels uncomfortable, slow, or frustrating… it may not be you.It may simply be the flange.

References (AMA Style)

1. Prime DK, Garbin CP, Visagg PW, et al. Nipple diameter changes during breast pumping: implications for breast shield design. Breastfeed Med. 2015;10(3):156–164.

2. Mesite Frem J. Babies in Common: Pump Flange Sizing Workshops and Observational Clinical Notes, 2020–2024.

3. Kent JC, Prime DK, Garbin CP. Principles for maintaining or increasing breast milk production. J Obstet Gynecol Neonatal Nurs. 2012;41(1):114–121.