Breast Pump Flange Fitting: Size, Shape, Comfort and Milk Removal

Choosing the right breast pump flange is often presented as simply finding the correct size. In reality, flange fitting is considerably more individualised than this.

The part of the breast pump that sits against the breast may be called a breast shield, flange or funnel, depending on the manufacturer and the terminology they use. These terms are often used interchangeably, although technically the component may contain both a flange, which sits against the breast, and a tunnel through which the nipple moves.

A good flange fit considers more than nipple diameter. It takes into account the size and shape of the nipple, the shape and geometry of the funnel, the pump being used, the reason for pumping, comfort and how effectively milk is being removed.

This matters because a flange can appear to fit when the pump is stationary but behave very differently once pumping begins.

Anatomy provides information. Function provides context.

What is flange fitting?

Flange fitting is the process of identifying the combination of flange size, shape, material and pump configuration that allows the nipple to move comfortably within the tunnel while supporting effective milk removal.

The aim is not simply to find a measurement that matches the nipple.

The aim is to find a combination that works.

Research increasingly supports this more individualised approach. A 2025 pilot study comparing traditional flange sizing with the Flange FITS™ Guide found that participants using smaller-fit flanges had both greater comfort and greater milk yield than when using their usual standard-sized flanges (Anders, Frem and McCoy, 2025).

The study is particularly interesting because the average nipple-tip measurement was 12.63 mm, while the standard manufacturer flanges used by participants were 21 mm and 24 mm. The smaller-fit flanges available for the study were 13, 15, 17 and 19 mm, with 15 mm being the most commonly selected size (Anders, Frem and McCoy, 2025).

The 12.63 mm figure is a research measurement, not a flange size that can simply be purchased. Flanges are produced in specific increments, so the measurement provides a starting point rather than an automatic answer.

The smaller-fit group produced an average of 153.2 g of milk compared with 138.2 g using the standard-fit flanges, while comfort scores were also significantly better (Anders, Frem and McCoy, 2025).

This does not mean that everyone should use a smaller flange. It demonstrates why traditional assumptions about flange sizing should not be applied without considering the individual.

Size is about the nipple

When we talk about flange size, we are talking primarily about the diameter of the nipple and the diameter of the tunnel through which the nipple moves.

This is different from the overall size of the breast.

The nipple should be able to move comfortably within the tunnel during pumping. It should not rub against the sides of the tunnel, and the flange should not draw areola into the tunnel.

The NHS similarly advises that a breast pump should not cause bruising or catch the nipple as it is drawn into the funnel, and recognises that different funnel sizes may be required to fit different nipples (NHS, 2026).

However, nipple dimensions are not necessarily fixed.

Research has demonstrated measurable changes in nipple diameter and length following milk expression. In one study, nipple horizontal diameter increased by approximately 2.25 mm, vertical diameter by approximately 1.64 mm and nipple length by approximately 2.27 mm following expression (Prime et al., 2023).

Nipple dimensions can therefore change with milk removal and other physiological factors, including changes associated with temperature. A measurement taken at one point in the day should not automatically be treated as a permanent measurement.

It is also entirely possible for the two nipples to be different sizes.

For this reason, there is no requirement for both breasts to use the same flange size.

Shape is not the same as size

Size and shape are two different considerations.

Size relates primarily to the diameter of the nipple tunnel.

Shape relates to the geometry of the funnel itself: how wide or narrow it opens, the angle of the flange, the depth and length of the tunnel, and how the funnel interacts with the breast.

This distinction is important because two funnels can have the same tunnel diameter but feel and perform very differently.

Different breast shapes and tissue characteristics may respond differently to different funnel geometries. Some designs have a narrower, more traditional conical profile, while others have a wider opening angle or a shallower profile.

Research supports the importance of this distinction. A randomised controlled trial comparing breast shields with 90° and 105° opening angles found that the wider 105° shield resulted in greater breast drainage, greater milk volume expressed, improved comfort and improved perceived fit (Sakalidis et al., 2020).

Shape is therefore not simply an aesthetic variation between manufacturers. Funnel geometry can influence how the breast is loaded during pumping and how effectively milk is removed.

There is more than one funnel shape

Manufacturers are beginning to recognise that a single funnel geometry cannot accommodate every breast.

For example, Maymom currently produces a range of MyFit™ funnels with different designs, including Classic, Crater, Saucer, Comfy and Pano. These differ in opening angle, profile, tunnel length, materials and the way the funnel sits against the breast (Maymom, 2026).

This illustrates why simply changing the numerical size of a flange may not solve a fitting problem.

If the tunnel diameter is appropriate but the funnel geometry does not suit the breast, changing the size alone may not produce the desired result.

The question is therefore not simply "What size flange do I need?" but "Which flange size and shape work with my anatomy and my pump?"

The Flange FITS™ Guide

The Flange FITS™ Guide represents an important development in moving away from traditional flange-sizing assumptions.

The guide provides a structured approach to measuring the nipple and then assessing the fit dynamically during pumping. In the Anders, Frem and McCoy (2025) study, the process began with measuring the nipple tip, selecting the closest available flange size and then trialling sizes approximately one to two sizes above and below that measurement while pumping.

The final selection was informed not only by measurement but also by what happened during pumping, including comfort and milk flow.

This is an important distinction.

The measurement provides a starting point; the fitting process determines what actually works.

The study also highlighted that many commercially available pumps have traditionally provided larger standard flange sizes, with 21 mm and 24 mm being common options. The authors noted that empirical support for traditional sizing approaches has been limited and that many major manufacturers have historically offered few options below 21 mm (Anders, Frem and McCoy, 2025).

This is beginning to change, but pump manufacturers have not necessarily caught up with the growing evidence around individualised fitting.

What does a correctly fitted flange look and feel like?

During pumping, the nipple should comfortably glide within the tunnel.

Pumping should not be painful.

The nipple should not repeatedly rub against the sides of the tunnel, become compressed or show signs of trauma.

The flange should not draw areola into the tunnel.

The breast should form an effective seal with the flange without excessive pressure.

Milk should be able to flow effectively and the pumping experience should be sustainable.

The NHS advises starting an electric pump at a low suction setting and increasing gradually, as using high suction immediately may cause pain or damage to the nipple (NHS, 2026).

A flange that is technically the correct diameter can still be unsuitable if the shape, positioning, suction or pump settings are not appropriate.

Comfort and effective milk removal

Comfort and effective milk removal should be considered together.

It is not useful to have a flange that produces a large volume of milk if it causes significant pain or nipple trauma. Equally, a flange that feels comfortable but results in poor milk removal may not be meeting the needs of someone who relies on pumping.

The research into both flange size and flange design demonstrates that these factors can affect milk removal as well as comfort (Anders, Frem and McCoy, 2025; Sakalidis et al., 2020).

This is particularly important for anyone who is pumping frequently, exclusively expressing or using pumping as part of a plan to increase milk production.

What if the aim is to increase milk production?

Flange fitting can be one part of supporting milk production, but it should not be viewed in isolation.

Milk production is influenced by the frequency and effectiveness of milk removal, the physiological response to pumping, pump settings, the individual's circumstances and a range of other factors.

If pumping is uncomfortable or milk is not being removed effectively, reviewing the flange may be appropriate.

However, a fitting problem is not necessarily the explanation for every low-volume pumping experience.

A flange fitting should therefore form part of an assessment rather than become the automatic explanation for reduced pumping output.

What about silicone flanges?

Silicone and other softer materials are increasingly available as alternatives to traditional rigid plastic funnels.

Some silicone products are marketed specifically for people with "elastic nipples".

The term "elastic nipple" is not consistently or universally defined, so it should be used cautiously rather than treated as a clearly defined clinical diagnosis.

In practice, some people find silicone more comfortable, while others do not.

There is also a distinction between marketing claims and clinical experience. In my own clinical practice, I have observed that rigid plastic funnels can sometimes produce better pumping outcomes than silicone alternatives, even where silicone has been marketed as being particularly suitable for "elastic nipples".

This is a clinical observation rather than a conclusion established by high-quality comparative research, and individual responses vary.

Material therefore needs to be considered alongside size, shape and pump function rather than treated as a universal solution.

The pump matters too

A flange does not operate independently of the pump.

Different pumps have different vacuum characteristics, cycle patterns and operating mechanisms. The same flange may therefore behave differently when used with different pump systems.

Pump settings matter too.

A higher suction setting is not automatically a more effective setting. The NHS recommends starting at a low suction level and increasing gradually, while remaining within a comfortable range (NHS, 2026).

The most effective setting is not necessarily the strongest setting. It is the setting that supports comfortable and effective milk removal for that individual.

Replacement parts are not the same as flange fitting

A flange can be correctly fitted and the pump can still perform poorly if other components of the pumping system are worn or damaged.

Valves, membranes, connectors and other pump components can deteriorate with use. Tubing can also become contaminated or damaged.

These components should therefore be checked as part of maintaining pump efficiency, but they should not be confused with flange fitting.

A correctly fitted flange cannot compensate for a worn valve, damaged membrane or other component that is affecting the performance of the pump.

If milk or condensation enters tubing, or if tubing becomes contaminated or damaged, the manufacturer's instructions should be followed regarding cleaning or replacement.

Cleaning and sterilising

Cleaning and sterilising are separate considerations from flange fitting, but they remain essential when expressing human milk.

The NHS advises that pumps and their components should be clean and sterilised before use and that the pump and parts should be washed and sterilised after expressing (NHS, 2026).

Always follow the manufacturer's instructions because different pump systems have different requirements and not every component is designed to be sterilised in the same way.

Tubing does not normally come into contact with milk and is therefore not routinely sterilised in the same way as milk-contact components. If milk, moisture, mould or other contamination enters the tubing, the manufacturer's instructions should be followed and replacement may be necessary.

The Parent Home FITS™ Guide

The Flange FITS™ approach is not intended to stop at the point where a flange has been selected.

The Parent Home FITS™ Guide provides practical support for caregivers to continue assessing their pumping experience at home, with particular attention to comfort and milk removal.

This is important because flange fitting is not necessarily a one-time event.

Nipple dimensions can change. Breast tissue can change. Pumping patterns can change. A person may also move between different pumps or different pumping situations.

The flange that worked well several months ago may therefore not remain the best option indefinitely.

The purpose of an individualised fitting process is not simply to give someone a flange size. It is to help them understand what a good fit looks and feels like so that they can recognise when something changes.

When flange fitting is not the answer

Not every pumping problem is caused by flange size or shape.

Pain, low milk volume, reduced milk removal, nipple trauma or difficulty expressing may have multiple contributing factors.

The pump itself, suction and cycle settings, breast fullness, milk ejection, pumping technique, frequency of expression and individual physiology may all need to be considered.

This is why a flange fitting should not be treated as a quick online measurement exercise.

For someone experiencing ongoing difficulties, an assessment by an IBCLC with specific lactation and pumping expertise can provide a more individualised evaluation of the whole pumping system.

The goal is not to sell a smaller flange. The goal is to identify what allows this individual to pump comfortably and remove milk effectively.

Moving beyond the traditional 24 mm flange

For many years, 24 mm has been presented as a standard flange size supplied with breast pumps.

That does not mean that 24 mm is the correct size for everyone.

The research underpinning the Flange FITS™ Guide demonstrates how different the individual measurements of nipples can be and how much difference flange selection may make to comfort and milk removal (Anders, Frem and McCoy, 2025).

The average nipple-tip measurement in that study was 12.63 mm, while the smaller-fit flanges available to participants ranged from 13 to 19 mm. The most frequently selected smaller-fit size was 15 mm (Anders, Frem and McCoy, 2025).

This is one reason why a manufacturer's supplied flange should be viewed as a starting point rather than proof that it is the correct size.

The same principle applies to shape.

A 24 mm flange with one funnel geometry may behave very differently from a 24 mm flange with another.

Flange fitting is individual

There is no single flange size that works for everyone.

There is no single funnel shape that works for everyone.

There is no single material that works for everyone.

And there is no guarantee that the same flange will remain appropriate throughout the expressing journey.

A useful fitting process considers:

• nipple diameter

• nipple shape

• changes during pumping

• left and right differences

• funnel geometry

• breast shape and tissue

• flange material

• pump compatibility

• pump settings

• comfort

• milk flow

• milk removal

• the reason the person is pumping

• and the condition of the wider pump system

The emerging evidence around the Flange FITS™ Guide reinforces the importance of moving away from a one-size-fits-all approach (Anders, Frem and McCoy, 2025).

Anatomy provides information. Function provides context.

For anyone experiencing ongoing difficulties with pumping, an individualised assessment can be far more useful than simply measuring the nipple and ordering the nearest available flange.

The right fit is the one that works.

References

Anders, L.A., Frem, J.M. 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.

Maymom (2026) MyFit™ breast pump flange/funnel range. Available from: Maymom. [Accessed 3 September 2026].

NHS (2026) 'Expressing and storing breast milk'. NHS. Available from: NHS website. [Accessed 3 September 2026].

NHS (2026) 'Expressing breast milk with a pump'. NHS Best Start in Life. Available from: NHS website. [Accessed 3 September 2026].

Prime, D.K. et al. (2023) 'Human milk expression technologies: an evaluation of efficacy and comfort of hands-free, in-bra, breastmilk collection pump set', Clinical Nutrition Open Science, 49, pp. 1–14.

Sakalidis, V.S. et al. (2020) 'Breast shield design impacts milk removal dynamics during pumping: A randomized controlled non-inferiority trial', Acta Obstetricia et Gynecologica Scandinavica, 99(11), pp. 1561–1567. doi: 10.1111/aogs.13897.