Pumping tips

"Room to Move”: A Step-by-step Guide to Measure Your Breast Pump Breast Shield Size

Time to read: 1 min.

Knowing how to measure your nipples and find the right flange size is everything when it comes to comfortable pumping and optimal milk flow. Pumping should also never be painful. That’s why Medela offers various flange sizes.

Content

Before You Measure: Why Sizing Is More Than a Number

Parts of the breast shield and inserts

How do I measure my flange size so that my breast pump works?

Before You Measure: Why Sizing Is More Than a Number

Measuring your nipple can be a helpful way to get started with breast shield sizing. It gives an initial reference point and helps narrow down options. But it’s important to know that measuring alone doesn’t determine your final fit. That’s because breasts and nipples are not static. During pumping, nipples naturally expand, lengthen, and change shape under suction. This means that a size based only on how your nipple measures before pumping may feel very different once pumping begins.

For this reason, we see measuring as the first step — not the final decision.

Medela’s ‘Room to Move’ is an evidence-based approach to breast shield sizing built around starting safely and allowing room for natural movement. Beginning with space to accommodate expansion helps reduce unnecessary pressure or rubbing during pumping, which can be especially important if you’re fitting yourself without hands‑on clinical guidance.

The most important part of fitting happens while you are pumping — paying attention to comfort, how freely the nipple moves, and how pumping feels over time. That’s why breast shield sizing should be seen as a process, not a one‑time measurement. This sizing approach serves as a flexible guideline rather than a strict rule. The measuring step will help you get started. From there, your body’s response during pumping, including comfort and effective milk flow, will guide you to the best fit.

Medela’s ‘Room to Move’ approach is a safe, evidence-based starting point for moms to select the best fit breast shield tunnel size when pumping.

Provides space for the natural expansion of the nipple with pumping

- The nipple increases on average 2-3 mm in both in diameter and length between pre- and post-pumping^1-3
- In some cases, expansion may range from minimal up to 8 mm^1-5

Minimizes nipple friction and tissue compression (enhancing comfort and safety)⁶
Accommodates natural biological variation between moms and breasts^1-6

Medela’s sizing approach serves as a flexible guideline rather than a strict rule, acknowledging that individual comfort and effective milk transfer ultimately determine the best fit.

Parts of a breast shield and insert

PersonalFit Flex™ breast shield

Medela PersonalFit Flex™ breast shield’s soft rim, wide opening angle and oval shape promote comfortable pumping and optimised milk supply.

Find out more

How do I measure my breast shield size so that my breast pump works?

How do I measure my breast shield size so that my breast pump works? To find your ideal breast shield size, start by measuring your nipple. Simply follow the instructions below.

1. To measure your nipple size, use a ruler or measuring tape to figure out the diameter size of your nipple at the base (across middle) in millimeters (mm). Remember not to include your areola.

2. Based on your measurement, you can determine your starting breast shield size by adding 4 mm. You can then choose the breast shield/insert tunnel that is closest to that resulting number.

Example: If your nipple measures 15 mm in diameter, add 4 mm. That gets you to 19 mm. The closest available starting breast shield size is 18 mm.

Inserts available: 15 mm, 18 mm, 21 mm sizes, to be used with 24 mm breast shield

3. Test your starting breast shield size.
Center the nipple and gently hold (do not push) the flange against your breast. Select your maximum comfort vacuum pressure level (MCV) on your pump.
This is a correct fit if, when the nipple is centered,
• it is comfortable during pumping
• the nipple moves freely
• milk is flowing freely after milk let-down (milk ejection)

Watch outs

The breast shield is too small if the nipple rubs along the side of the tunnel, or nipple is red, sore or painful. If this is the case, try a bigger size.
The breast shield is too large if the nipple and too much areola are pulled into the tunnel. If this is the case, try a smaller size.

After pumping, check your breast and nipples. Good milk removal is indicated by breasts that feel soft and comfortable. Milk volume can vary depending on your stage of lactation and whether or how often your baby is breastfeeding.

Re-evaluate after every pumping session to check that you have the best fit for comfortable and effective milk removal.

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Additional information

Finding the right breast shield: Key to comfortable and effective pumping article

FAQs

What happens if I use the wrong breast shield size?
Using the wrong breast shield size can lead to discomfort, pain, and reduced milk output. A poor fit may also cause clogged ducts or decreased milk supply over time.

To ensure comfortable and effective pumping, it's important to find the right breast shield size. Regularly checking your fit and adjusting if needed can help improve your pumping experience.

How do I know if my breast shield size is too big or too small?
• If the breast shield is too small, your nipple may rub against the sides, causing irritation or even damage.
• If it's too large, too much of your areola may be pulled in, which can lead to inefficient milk removal and soreness.

Can my breast shield size change over time?
Yes, that’s possible. Your breasts can change between the third trimester and after childbirth and that includes your nipples. They may also continue to change throughout your pumping journey, so it's important to check your breast shield size regularly.

Is it possible I need a different breast shield size for each breast?
Yes, it's possible! Each breast is unique, and nipple sizes can vary between them. Some moms find that they need a different breast shield size for each breast to ensure a comfortable fit and optimal milk flow.

To get the best pumping experience, measure each nipple separately and check if both require the same size or different ones, as one breast may produce more milk than the other and breasts/nipples are often not symmetrical. If you're experiencing discomfort or uneven milk output, trying a different breast shield size for each breast may help.

Why does pumping hurt? Could it be the wrong breast shield size?
Pumping should never be painful. If you experience pain, discomfort, or nipple soreness, it could be due to an incorrect breast shield size.

Other factors, such as high suction settings, improper breast shield positioning, or engorgement, can also cause pain. To improve your comfort, try adjusting your breast shield size, adjust suction levels, and ensuring

proper nipple alignment in the breast shield.

If pain persists, consult a lactation specialist for further guidance.

What sizes does Medela offer?

PersonalFit Plus: Available breast shield sizes are 18 mm, 21 mm, 24 mm, 27 mm, 30 mm, and 36 mm.
PersonalFit Flex: Available breast shield sizes are 18 mm, 21 mm, 24 mm, 27 mm, 30 mm, and 36 mm.
Hands-Free In-Bra: Available breast shield sizes are 18 mm, 21 mm, 24 mm, and 27 mm.
Inserts: Available tunnel sizes are 15 mm, 18 mm, and 21 mm.

Inserts FAQs

Can the inserts be used with all Medela Personal Use Pumps?
No. Medela inserts can only be used with selected compatible Medela breast pumps and Medela breast shields, so that performance and efficacy is maintained. This information is available in the insert instructions for use.

Compatible Personal Use Pump breast shields include Hands-free, PersonalFit Flex™, and PersonalFit™ PLUS.
Compatible Medela Breast pumps include Harmony™, Motion InBra™, Swing Maxi™, Solo™, Freestyle™, Pump In Style^®.

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References

1 Fortin Higgins A. Flange Sizing Recommendations for Frequent Breast Pump Use. Clin Lactation. 2022; 13(3):159–169.
2 Prime DK. Dynamics of milk flow and milk ejection during breast expression in women [PhD Thesis]: The University of Western Australia; 2010.
3 Gridneva Z et al. Efficacy Assessment of the Breast Shield Size. Proceedings. 2025; 112:7.
4 Rusby JE et al. Breast duct anatomy in the human nipple: three-dimensional patterns and clinical implications. Breast Cancer Res Treat. 2007; 106(2):171–179.
5 Kujawa-Myles S et al. Maternal intravenous fluids and postpartum breast changes: a pilot observational study. Int Breastfeed J. 2015; 10:18.
6 Jones E, Hilton S. Correctly fitting breast shields are the key to lactation success for pump dependent mothers following preterm delivery. J Neonatal Nurs. 2009; 15(1):14–17.

"Room to Move”: A Step-by-step Guide to Measure Your Breast Pump Breast Shield Size

Before You Measure: Why Sizing Is More Than a Number

Parts of a breast shield and insert

How do I measure my breast shield size so that my breast pump works?

Related Products

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