Breast Pump Flange Fitting: Finding What Works for You
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Choosing a breast pump flange is not simply about finding a size that feels comfortable. The right fit should support comfortable milk expression, effective milk removal and a good milk yield, while also working with the individual breast, nipple, pump and the circumstances in which expressing takes place.
Flange fitting has also changed in recent years. Traditional advice has often been based on measuring the nipple at its base and then adding a specified number of millimetres to determine the flange size. More recent guidance and research have moved towards measuring the nipple tip and using that measurement as a starting point for assessing fit during pumping.
The Flange FITS™ Guide is one example of this approach. There is a guide for parents and separate professional guidance, and the approach focuses on assessing the flange during pumping rather than relying on measurement alone.
A flange fit assessment should therefore ask a much broader question than:
> “What size flange do I need?”
It should ask:
> “What flange, pump settings and expressing approach allow this individual to express comfortably and effectively?”
What is a flange fit assessment?
A flange, also called a breast shield, is the part of a breast pump that sits against the breast and surrounds the nipple. During pumping, the nipple is drawn into the flange tunnel.
Although flange fitting is often discussed in terms of millimetres, diameter is only one part of the assessment.
A comprehensive assessment considers:
nipple size and shape
how the nipple moves within the tunnel during pumping
whether the nipple can glide comfortably within the tunnel
how much surrounding breast tissue is drawn into the tunnel
changes in nipple colour, shape or swelling
breast and nipple comfort
the flange's shape and material
pump suction and cycle settings
milk flow and milk yield
expressing frequency and duration
the pump being used
whether the two breasts respond differently
previous experiences with pumping
and the person's individual goals for expressing.
Where appropriate and with consent, observing part of a pumping session can provide information that cannot be obtained from measurement alone.
The aim is not to identify one universally “correct” flange size. It is to find an arrangement that works for that individual.
Start with the history, not the measurement
Before measuring a nipple, it is important to understand why the person is expressing.
For some, pumping may be occasional and practical. For others, expressing may be central to maintaining their milk production because their baby is not currently transferring milk effectively at the breast. Others may be exclusively expressing, combination feeding, returning to work, expressing during separation from their baby, or expressing for another individual reason.
These different circumstances mean that the goals of expressing can be very different.
A useful assessment therefore explores:
What are you hoping to achieve through expressing?
How often are you currently pumping?
How long does each session take?
What pump are you using?
Which flange or flanges are you currently using?
What pump settings are you using?
How much milk are you typically expressing?
Does milk flow easily?
Does milk flow change during the session?
Do you experience discomfort or nipple changes?
Have you already tried different flange sizes or designs?
Does one breast respond differently from the other?
Has anything changed recently?
This history matters because a flange cannot be assessed in isolation from the person using it.
Someone who is expressing once a day has different requirements from someone expressing eight times a day. Someone trying to maintain milk production while their baby temporarily feeds elsewhere has different priorities from someone expressing occasionally after a breastfeed.
Individual goals should therefore form part of the flange assessment.
Nipple measurement is a starting point — not a prescription
Nipple measurements can be useful, but they are not fixed measurements.
Nipple diameter can change throughout the day and can be influenced by factors including temperature, stimulation, lactation stage and what has happened immediately beforehand. Nipple tissue can also change during and after pumping.
The left and right nipples may not be the same size either. It is therefore important to assess each side individually rather than assuming that one flange size will necessarily work for both breasts.
Current approaches to flange fitting have also moved away from automatically adding a predetermined number of millimetres to a nipple measurement.
The Flange FITS™ Guide recommends measuring the nipple tip, with the nipple gently stimulated or tugged so that it is everted, and using that measurement as a starting point. It then recommends trying nearby flange sizes during pumping to identify which provides the best combination of comfort and milk flow. https://www.uhs.nhs.uk/Media/UHS-website-2019/Docs/Services/Maternity/The-Flange-FITS-Guide-for-optimal-comfort-efficiency-and-milk-yield.pdf
This is an important change from older approaches that commonly measured the nipple at its base and added a fixed number of millimetres.
A recent comparative pilot study found that smaller-fit flanges selected using the Flange FITS™ approach resulted in greater comfort and, on average, greater milk output than the standard sizing approach used in the study. Importantly, the researchers concluded that flange fitting should be individualised and may require trialling more than one size. https://pubmed.ncbi.nlm.nih.gov/39614713/
This does not mean that everyone should automatically use a smaller flange.
It means that measurement should be used as a starting point for assessment rather than as a rigid prescription.
What should happen inside the tunnel?
During pumping, the nipple should be positioned centrally and should be able to glide comfortably within the tunnel.
The sides of the nipple may gently contact the tunnel walls, but the nipple should not be tightly compressed or repeatedly rubbing against the tunnel.
Signs that the fit may need reassessment can include:
rubbing or friction
blanching or marked colour change
swelling during pumping
nipple trauma
discomfort or pain
restricted nipple movement
excessive areolar tissue being drawn into the tunnel
reduced or interrupted milk flow.
The Flange FITS™ Guide describes an optimal fit as one where the nipple gently glides within the tunnel, pumping is comfortable and milk comes out easily. https://www.uhs.nhs.uk/Media/UHS-website-2019/Docs/Services/Maternity/The-Flange-FITS-Guide-for-optimal-comfort-efficiency-and-milk-yield.pdf
This is why simply looking at a measurement chart cannot replace observing what happens during pumping.
Flange shape matters too
Flanges are not simply different sizes of the same shape.
Different manufacturers produce breast shields with different tunnel lengths, angles, contours and materials. Some have a more traditional rigid design, while others use softer silicone or different breast-shield shapes.
Maymom, for example, now offers several breast-shield designs with different shapes and dimensions, illustrating why two flanges with the same stated tunnel diameter may not feel or perform in the same way. https://www.maymom.com/index.php/products-menu/breastshield-selection-guide
This is particularly relevant when a person has tried several sizes but still does not feel that their flange is working well.
A flange fitting service should therefore involve trying different flange sizes and, where appropriate, different flange shapes and designs, rather than simply measuring and recommending one size.
The material can also make a difference. Some silicone flanges and in-bra pump inserts may benefit from a small amount of suitable lubrication to reduce friction. The Flange FITS™ Guide specifically notes that a thin layer of coconut oil or nipple balm can increase comfort, and some silicone-flange manufacturers provide specific guidance on lubrication. https://www.uhs.nhs.uk/Media/UHS-website-2019/Docs/Services/Maternity/The-Flange-FITS-Guide-for-optimal-comfort-efficiency-and-milk-yield.pdf
Any product used should be compatible with the equipment and appropriate for use around the nipple and breast, and manufacturer instructions should be followed.
Pump settings matter
Finding the right flange does not necessarily solve every pumping problem.
Pump settings also influence the experience and effectiveness of expressing.
Many pumps have a stimulation or let-down mode followed by an expression mode. However, relying on the pump's automatic programme for the entire session does not necessarily provide the best response for every individual.
It can be useful to move between stimulation and expression modes during a session, depending on what is happening with milk flow.
For example, if milk flow slows after an initial let-down, returning temporarily to a faster stimulation pattern may help stimulate another milk ejection before returning to a slower expression pattern.
The aim is not simply to use the highest suction or follow a predetermined programme from beginning to end. Pump settings can be adjusted according to comfort, milk flow and the individual's response.
Research has demonstrated that vacuum level influences milk flow and yield, with maximum comfortable vacuum producing greater milk removal than softer settings in one study. This supports the principle of finding an effective level that remains comfortable rather than simply choosing the strongest possible suction. https://pubmed.ncbi.nlm.nih.gov/18333764/
Pumping is not the same as breastfeeding at the breast
The volume obtained during a pumping session should not be interpreted as though it were a direct measurement of how much milk the breast can produce for a baby feeding at the breast.
Breastfeeding involves a complex physiological and hormonal process, including infant suckling, sensory stimulation and neuroendocrine responses that contribute to milk ejection.
A pump provides a mechanical stimulus, and its effectiveness depends on the pump, flange, settings, timing, breast fullness, milk ejection response and the individual's response to the equipment.
Consequently, the amount expressed by a pump does not necessarily represent the amount of milk that would be available to, or removed by, a baby feeding directly at the breast.
This distinction is particularly important when supporting someone who is worried that a low pumping volume automatically means they have low milk production.
Pump output needs to be interpreted in context.
Comfort matters — but yield and milk removal matter too
Comfort is an important part of flange fitting, but it should not be the only outcome considered.
A person may experience comfortable pumping but obtain very little milk. Another may obtain a reasonable volume but develop swelling or nipple trauma. Someone else may find that one breast responds well to a particular flange while the other requires a different size or shape.
A useful assessment therefore considers:
Is it comfortable?
Is the nipple moving appropriately?
Is the breast shield working with the person's anatomy?
Is milk flowing effectively?
Is milk being removed efficiently?
Is the set-up sustainable for the person's expressing goals?
This is the value of a proper flange fit assessment: it considers the whole expressing experience rather than treating flange size as an isolated measurement.
Why professional assessment can be valuable
There is a growing amount of information online about flange sizing, but self-measurement and self-selection cannot always identify why pumping is difficult.
An IBCLC has advanced education and clinical experience in lactation and breastfeeding care. The IBCLC credential requires substantial lactation-specific education and clinical practice in addition to passing an international examination, and IBCLCs work within defined professional competencies and scope of practice. https://iblce.org/about-iblce/
For flange fitting specifically, it is also worth asking whether the professional has undertaken advanced education in breast pump and flange fitting and has practical experience observing pumping sessions and working with different pumps, flange sizes, shapes and materials.
This is important because flange fitting is not simply something that can be learned from a manufacturer's size chart or through occasional self-study.
A skilled clinician should be able to consider the flange alongside the wider lactation picture and recognise when a pumping difficulty may require further assessment or referral.
When flange fit may not be the whole explanation
Changing the flange can be helpful, but not every pumping problem is caused by flange size.
Persistent nipple pain can have a range of possible causes, including trauma, dermatitis, infection, vasospasm and pumping-related mechanical problems. A careful assessment is preferable to repeatedly changing flange sizes without considering the wider picture.
Similarly, a low expressed volume does not automatically mean low milk production.
Pump output can be influenced by:
timing of the pumping session
breast fullness
milk ejection
frequency of milk removal
pump effectiveness
flange fit
pump settings
stress and comfort
individual response to the pump
and the reason the person is expressing.
If breastfeeding is also part of the feeding plan, it may be important to consider breast and nipple comfort, infant positioning and attachment, milk transfer and infant feeding function.
Anatomy provides information. Function provides context.
What happens during a flange fitting service?
A flange fitting service should be more than measuring a nipple and recommending a size.
The assessment may involve:
1. Taking a detailed history and establishing the person's expressing goals.
2. Assessing each nipple individually.
3. Measuring the everted nipple tip as a starting point.
4. Observing pumping where appropriate and with consent.
5. Trialling different flange sizes.
6. Trying different flange shapes and designs where indicated.
7. Reviewing suction and cycle settings.
8. Considering stimulation and expression modes and how the person responds to them.
9. Assessing comfort, nipple movement, breast tissue movement and milk flow.
10. Agreeing a practical plan that the person can realistically use at home.
11. Reviewing the response and making further adjustments where necessary.
The outcome may be a different flange size, a different shape or material, a different pump setting, a change in pumping technique — or a combination of these.
Sometimes the assessment may also identify that the flange is not the main problem.
When to seek further support
Please seek prompt medical advice for fever or flu-like symptoms, a hot or increasingly painful area of the breast, rapidly worsening redness, pus, significant bleeding or severe pain.
Further lactation support may also be appropriate when pumping remains uncomfortable, nipples are repeatedly damaged, milk expression changes unexpectedly, or there are concerns about an infant's feeding, hydration, growth or wellbeing.
Flange fitting is most useful when it forms part of skilled, individualised lactation care rather than being treated as a quick equipment fix.
There is no single flange size that is right for everyone.
Measure as a starting point. Observe what happens during pumping. Consider the shape and material of the flange. Listen to the person's goals and experience. Then assess comfort and effective milk removal together.
That is the difference between simply choosing a flange size and actually assessing flange fit.
References
Academy of Breastfeeding Medicine (2016) ‘ABM Clinical Protocol #26: Persistent pain with breastfeeding’, Breastfeeding Medicine, 11(2), pp. 46–53.
Anders, L.A., Mesite Frem, J. and McCoy, T.P. (2025) ‘Flange Size Matters: A Comparative Pilot Study of the Flange FITS™ Guide Versus Traditional Sizing Methods’, Journal of Human Lactation, 41(1), pp. 54–64. doi:10.1177/08903344241296036.
Kent, J.C., Prime, D.K. and Garbin, C.P. (2012) ‘Principles for maintaining or increasing breast milk production’, Journal of Obstetric, Gynecologic & Neonatal Nursing, 41(1), pp. 114–121.
Mesite Frem, J. (2023) The Flange FITS™ Guide for Optimal Comfort, Efficiency and Milk Yield. Babies in Common.
Mitoulas, L.R., Lai, C.T., Gurrin, L.C., Larsson, M. and Hartmann, P.E. (2002) ‘Efficacy of breast milk expression using an electric breast pump’, Journal of Human Lactation, 18(4), pp. 344–350.
Prime, D.K., Geddes, D.T., Spatz, D.L., Robert, M. and Hartmann, P.E. (2011) ‘Using milk flow rate to assess milk ejection and breast milk removal during pumping’, Journal of Human Lactation, 27(2), pp. 159–166.
Prime, D.K., Garbin, C.P., Hartmann, P.E. and Kent, J.C. (2012) ‘Simultaneous breast expression in breastfeeding women is more efficacious than sequential breast expression’, Breastfeeding Medicine, 7(6), pp. 442–447.
Witt, A.M., Bolman, M., Kredit, S. and Vanic, A. (2016) ‘A systematic review of the effects of breast milk expression methods on milk volume and milk composition’, Journal of Human Lactation, 32(3), pp. 495–504.
World Health Organization (2009) Infant and young child feeding: Model Chapter for textbooks for medical students and allied health professionals. Geneva: World Health Organization. ```
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