# Breast Pump Flange Sizing: Why the Default Size Is Usually Wrong > The 24 mm and 28 mm shields bundled with most pumps fit a narrow slice of nipple anatomy. Here is how to measure correctly, what sizes actually work for most people, and how silicone inserts solve the problem affordably. *Published 2026-06-25 · By Claire Bennett, CPST* The short answer The 24 mm and 28 mm flanges packaged with most pumps fit a narrow slice of nipple anatomy. Clinical IBCLC data show the most commonly needed sizes run from 13 mm to 21 mm. Measure your nipple tip diameter, trial one size down, and use silicone inserts from Maymom or Nenesupply to reach a correct fit without buying a new pump. Flange sizing is the single most impactful technical decision you make as a pumping parent — and the one most likely to be made incorrectly, because the default flanges in the box are chosen for inventory efficiency, not individual anatomy. Getting this one detail right can mean the difference between a comfortable, productive pumping session and weeks of pain, low output, and unnecessary frustration. ## Why do default pump flanges fit so few people? The **Spectra S1 Plus** ships with 24 mm and 28 mm flanges. Medela's current models include a 21 mm and 24 mm shield. These sizes represent the statistical middle of nipple anatomy as estimated by manufacturers — not the clinical reality that IBCLCs observe daily in practice. A 2025 peer-reviewed pilot study published in the [Journal of Human Lactation](https://journals.sagepub.com/doi/10.1177/08903344241296036) by IBCLC Jeanette Mesite Frem, Lisa A. Anders, and Thomas P. McCoy — the study that validated the Flange FITS method — found that in clinical populations, the most commonly needed flange sizes range from **13 mm to 21 mm**, substantially below the packaged defaults. Many pumping parents are using a flange one to two sizes too large without knowing it. The consequences of a poor fit are not minor. A flange that is too large draws areola tissue into the nipple tunnel alongside the nipple, reducing suction efficiency and causing bruising or soreness at the areola base. A flange that is too small creates friction against the nipple shaft — the nipple rubs the tunnel walls with every cycle, causing blistering and a characteristic white compression ring at the tip. In either case, the pump's suction is not translating into effective milk removal. Observational data from [Aeroflow Breastpumps](https://aeroflowbreastpumps.com/blog/best-breast-pumps) found that 82.9% of pumping mothers rate suction strength as the most important factor in pump selection — but optimal suction settings are irrelevant if the flange geometry is wrong. Fit is upstream of everything else. ## How do you measure for the right flange size? The measuring process takes about two minutes and requires only a millimeter measuring tape or a printed ruler. Here is the standard IBCLC-recommended method: - **Time it correctly.** Measure after a warm shower when nipple tissue is relaxed — not immediately after a feeding or pumping session when the nipple may be compressed or temporarily enlarged. - **Measure nipple tip diameter only.** Place the tape across the widest point of the nipple itself, not the areola. The areola is not part of this measurement. Record the diameter in millimeters. - **Apply the Flange FITS starting point.** The Flange FITS method, developed by Jeanette Mesite Frem and validated in the 2025 *Journal of Human Lactation* study, begins at the measured nipple diameter and tests one size smaller and one size larger to find peak comfort and output. Your measured size is a starting point, not a final answer. - **Assess the fit during a session.** During a pumping session, the nipple should move freely in and out of the tunnel without touching the sides. Minimal surrounding areola — ideally less than a few millimeters — should be drawn in. You should feel rhythmic pulling but no friction, pinching, or burning. A useful self-check After removing the flange, your nipple should look the same as it did before pumping — no white rings, no redness at the shaft, no bruising at the areola edge. Any of those marks is a reliable sign that the size needs adjustment. If self-measurement is unclear, an IBCLC lactation consultation includes a hands-on flange fitting assessment. These visits are **covered by most major insurance plans** — including Aetna, Cigna, UnitedHealthcare, and Anthem — as a preventive lactation benefit under the Affordable Care Act, so there is typically no out-of-pocket cost. The [Lactation Loop IBCLC guide](https://lactationloop.com/blog/breast-pump-flange-sizing-an-ibclcs-complete-guide) also provides a printable self-measurement template. ## What are silicone flange inserts, and when should you use them? Silicone inserts are soft inserts that slide inside a standard 24 mm flange and reduce the effective tunnel diameter. **Maymom** and **Nenesupply** are the two most established brands in this category; both offer inserts that reduce the tunnel opening to sizes from 13 mm to 21 mm. They are widely available for approximately $10–$20 per set — a fraction of the cost of a full pump replacement or upgrade. Inserts work because they borrow the structural body of the standard flange while providing a smaller, softer nipple tunnel. The medical-grade silicone material is also gentler against the nipple than the firmer polypropylene of most OEM flanges, which many users find adds a meaningful comfort benefit independent of the size change. Most Maymom and Nenesupply inserts are compatible with Spectra and Medela pump bodies; confirm the compatibility chart for your specific pump before ordering. For wearable pumps like the **Elvie Stride** or **Willow Go**, check the manufacturer's guidance on insert compatibility, as some cup geometries differ from traditional flange designs. Inserts do wear out. Replace them at the first sign of tearing, discoloration, or reduced suction — worn silicone pump parts are among the most common causes of sudden output decline. The general guidance for pump valves and membranes (replacement every two to four weeks under daily use) applies to inserts as well under comparable use conditions. ## A note on flange size and wearable pumps Flange fit matters even more with wearable pumps than with traditional bench-style pumps, for a simple reason: you cannot observe the nipple movement inside a wearable cup while it is tucked in your bra. With a traditional pump, you can glance down and immediately see whether areola is being drawn in or the nipple is rubbing the walls. With the Elvie Stride or Willow Go, that visual feedback is gone. A controlled study found no statistically significant difference in breast emptying or total milk volume between wearable and traditional pumps *when flange fit was correct*. But the same research noted that 10–50% output reductions were reported by some wearable-pump users outside clinical conditions — with poor flange fit identified as the primary cause. If your wearable pump seems to underperform relative to your traditional pump, flange sizing is the first variable to reassess, not suction settings or pump model. Most lactation professionals recommend establishing correct flange size using a traditional double-electric pump in the first four to six weeks postpartum — when you can observe the fit and have IBCLC support readily available — before adding a wearable pump for daytime mobility. Confirming fit in a visible, adjustable setting first removes a significant source of uncertainty when you transition to pumping hands-free. *This article provides general educational information about breast pump flange sizing. It is not a substitute for individualized medical or lactation advice. If you are experiencing pain while pumping, unexpectedly low output, or any concerns about milk supply, please consult a certified lactation consultant (IBCLC) or your obstetric provider.* ## Sources 1. [How to Measure Breast Pump Flange Size: An IBCLC's Complete Guide](https://lactationloop.com/blog/breast-pump-flange-sizing-an-ibclcs-complete-guide) 2. [Flange Size Matters: A Comparative Pilot Study of the Flange FITS Guide Versus Traditional Sizing Methods](https://journals.sagepub.com/doi/10.1177/08903344241296036) 3. [Wearable vs. Traditional Electric Breast Pumps: 2025 Comparison](https://www.annabella-pump.com/blogs/milk-our-knowledge/wearable-vs-traditional-electric-breast-pumps-in-2025-which-delivers-better-output) 4. [Best Breast Pumps](https://aeroflowbreastpumps.com/blog/best-breast-pumps) 5. [Elvie Stride vs Willow Go Comparison](https://bemybreastfriend.com/blogs/breast-pump-comparisons/elvie-stride-vs-willow-go-comparison) --- Source: https://natalnew.com/baby-gear/breast-pump-flange-sizing-guide Index: https://natalnew.com/llms.txt · Full text: https://natalnew.com/llms-full.txt