How a Breast Pump Works: A Complete Guide for New Moms

You’ve probably seen a breast pump in a store, on a registry list, or sitting in a box at home — but have you ever wondered what’s actually happening inside it? How does a device manage to do something a baby does instinctively?

If you’ve ever asked yourself “how does a breast pump work?” — you’re not alone. It’s one of the most common questions new and expecting moms have, and the answer is surprisingly fascinating once you break it down.

In this guide, we’ll walk through exactly what happens when you switch on a pump, how the different types work, and what factors affect how well it works for you. If you’re still at the very beginning, you may want to first read our guide on what a breast pump is before diving in here.

Before getting into the mechanics, it helps to understand what a breast pump is trying to replicate: a baby’s natural sucking pattern.

When a baby nurses, they don’t just suck continuously. They actually feed in two distinct phases:

1. Stimulation phase — fast, light sucking at the start of a feed to trigger the let-down reflex (the moment milk begins to flow)
2. Expression phase — slower, deeper sucking once the milk is flowing, to draw milk out efficiently

A breast pump is engineered to copy this exact two-phase rhythm. The stimulation phase uses quick, gentle suction pulses to signal your body that a feed is starting. Once let-down is triggered — that familiar tingling or rushing sensation — the pump shifts into expression mode: a slower, stronger suction cycle that draws the milk out.

This is why modern breast pumps have two separate modes. It’s not a gimmick. It’s directly mimicking what your baby’s mouth does naturally, because your body is designed to respond to that specific pattern.

You don’t need to memorise every component, but understanding the parts that directly affect the pumping action helps you use the pump more effectively. For a full breakdown of every part and what it does, see our breast pump parts explained guide. For now, here are the ones that matter most for how it works:

• Flange (breast shield): The funnel-shaped cup that fits over your nipple and areola. It creates an airtight seal — without a good seal, there’s no suction. Flange sizing is critically important and often overlooked. The wrong size reduces milk output significantly. Our guide on what a breast pump flange is and how to find your size covers this in detail.
• Tubing: Connects the motor to the flange, carrying the suction. Tubing should never have milk in it (more on that in a moment).
• Motor/pump unit: The engine of the whole operation. Generates the rhythmic negative pressure that creates suction.
• Valve and membrane: A small one-way valve that controls the direction of suction — it ensures suction pulls milk toward the bottle, not back into the tubing.
• Collection bottle: Where the expressed milk collects during the session.
• Backflow protector: A filter between the flange and tubing that prevents milk or moisture from entering the tubing or motor. This is what defines a closed system pump.

Here’s the full pumping process broken down into simple steps:

1. The flange is positioned over the nipple, creating an airtight seal around the nipple and areola
2. The motor activates, generating rhythmic negative pressure — controlled suction that pulses rather than holds
3. Stimulation phase begins — fast, light suction pulses mimic early nursing to trigger the let-down reflex
4. Let-down occurs — milk ejection reflex activates, and milk begins to flow from the milk ducts
5. The pump shifts to expression phase — suction slows down and deepens to draw milk out efficiently
6. The valve and membrane direct milk flow — one-way valves ensure milk travels toward the bottle, not backward
7. Milk flows through the flange into the collection bottle — gravity and suction work together
8. Session ends — milk is stored, pump parts are disassembled and cleaned ready for next use

Most pumps let you adjust both the speed (cycles per minute) and the suction strength (measured in mmHg — millimetres of mercury, the standard unit for suction pressure). Higher suction is not always better. The most comfortable and effective setting is usually a moderate suction level — the highest setting you can use comfortably, not the maximum possible.

An electric breast pump uses a motorised unit to automate the entire suction cycle. You plug it in or run it on battery, set your preferred speed and suction level, and the motor handles the rhythm.

The two-phase cycle (stimulation then expression) is built into the software or mechanics of the pump. Most modern electric pumps let you switch between the two phases manually, or they detect let-down and transition automatically.

Double electric pumps work both breasts simultaneously using a single motor and split tubing. This halves your pumping time and also tends to produce more milk per session, because stimulating both breasts at once triggers a stronger hormonal response.

Electric pumps vary widely in suction strength — hospital-grade pumps reach up to 300 mmHg, while standard personal-use pumps typically sit in the 150–250 mmHg range. For more on electric vs manual and which suits different situations, see our types of breast pumps guide.

A manual breast pump uses the exact same suction principle as an electric pump — the difference is that you provide the power instead of a motor.

Squeezing the handle compresses a small chamber inside the pump body, pushing air out. When you release the handle, the chamber expands and creates negative pressure — suction — that draws milk from the breast into the flange and down into the collection bottle.

The rhythm is entirely in your hands. Squeeze faster for the stimulation phase, then slow down and squeeze more firmly once let-down occurs.

This takes a little practice to get right, but many moms find manual pumps surprisingly effective — and some report getting more milk from a manual pump than an electric one, because they have complete control over the rhythm. We explore this more in our post on whether manual breast pumps really work.

Manual pumps are silent, require no power source, and are easy to travel with. The trade-off is that they are more physically tiring, especially for longer or frequent sessions.

Wearable breast pumps — like the Elvie, Willow, or Momcozy — put the entire pumping system inside a pod that fits directly in your bra. There are no external tubes, no bottles dangling, and no need to sit near a power outlet.

Inside the pod, a miniaturised motor generates the same rhythmic suction cycle as a traditional electric pump. Milk collects in an internal cup built into the pod itself, rather than an external bottle.

Most wearable pumps are controlled via a smartphone app or buttons on the device, and some can detect let-down and switch phases automatically.

The key trade-off: wearable pumps typically have lower maximum suction strength than plug-in electric pumps, because the motor has to fit inside a bra-sized unit. This makes them excellent for on-the-go sessions and maintaining supply during work, but many lactation consultants recommend using a stronger pump as your primary pumping tool if you rely heavily on pumping for your full milk supply. You can read a full breakdown in our guide on whether wearable breast pumps are worth it.

Silicone pumps — most famously the Haakaa — work on a completely different principle: passive suction, with no motor at all.

You squeeze the silicone bulb to push air out, then press the flange against your breast and release. The natural rebound of the silicone creates a gentle suction that holds the pump in place against your breast.

Unlike active pumping, you’re not operating a suction cycle. The silicone pump simply catches the milk that flows naturally during let-down — typically from the opposite breast while your baby feeds on the other side.

This makes silicone pumps excellent for building a freezer stash with minimal effort, but they are not a substitute for active pumping sessions. They won’t maintain or build supply the way a motorised pump does. For more on whether a Haakaa counts as a manual pump, see our dedicated Haakaa guide.

The mechanics of a pump are only part of the equation. How well it works for you also depends on several personal and practical factors:

Flange size is the single most important variable. If the flange is too small, it restricts the nipple and reduces milk flow. Too large, and suction is lost. Most pumps come with a standard 24mm or 28mm flange, but many moms need a different size. See our flange sizing guide for how to measure correctly.

Suction level matters, but more is not better. Research consistently shows that the most effective suction is the highest level that remains comfortable — often called “maximum comfort vacuum.” Pumping at painful levels doesn’t produce more milk and can cause nipple damage.

Pumping frequency signals your body how much milk to produce. The more consistently you pump, the stronger the hormonal signal to maintain supply. Skipping sessions, especially in the early weeks, can reduce supply quickly.

Relaxation and let-down play a bigger role than most people expect. Stress, anxiety, and discomfort can physically inhibit the let-down reflex, reducing what the pump is able to express — even if the pump itself is working perfectly. Looking at photos or videos of your baby, using a warm compress beforehand, or pumping in a quiet space can all help.

Pump condition and part age affect performance over time. Membranes and valves wear out and lose their seal, reducing suction even when the motor is fine. Most manufacturers recommend replacing small parts every 1–3 months with regular use. Our guide on how long breast pumps last covers when to replace parts and the pump itself.

Every breast pump — electric, manual, wearable, or silicone — works by replicating the rhythm your body is already primed to respond to. The two-phase suction cycle is at the heart of all of them. 

Understanding that simple principle makes everything else — the settings, the parts, the troubleshooting — much easier to figure out.

This guide reflects general breastfeeding and pumping knowledge and is not a substitute for advice from a certified lactation consultant (IBCLC) or your healthcare provider.

1. Ramsay, D.T. et al. (2004) — Ultrasound imaging of milk ejection in the breast of lactating women. Pediatrics, 113(2), 361–367. (Basis for two-phase sucking cycle research)
2. Kent, J.C. et al. (2008) — Response of breasts to different stimulation patterns of an electric breast pump. Journal of Human Lactation, 24(2), 116–124. (Suction patterns and milk output)
3. Prime, D.K. et al. (2012) — Simultaneous breast expression in breastfeeding women is more efficacious than sequential expression. Breastfeeding Medicine, 7(6), 442–447. (Double pumping efficiency)
4. Meier, P.P. et al. (2016) — Breast pump suction patterns that mimic the human infant during breastfeeding. Journal of Perinatology, 36, 127–132. (Motor-mimicry in pump design)
5. ABM Clinical Protocol #8 (2017) — Human Milk Storage Information for Home Use for Full-Term Infants. Academy of Breastfeeding Medicine. (General pumping and storage guidelines)
6. La Leche League International — Using a Breast Pump. llli.org (General guidance on pump mechanics and use)
7. Cleveland Clinic — Breast Pumping: What It Is, Benefits & How To. my.clevelandclinic.org (Clinical overview of breast pump function)