Shield sizing 101: why 99% of mums need a smaller shield

Our top-selling shield is 24mm. And we hate that for you.

Every Spectra ships with a 24mm shield in the kit. This is a good starting point, but it's very, very unlikely to be the right size for you. It's big enough that everyone can use it, but once you're properly sized we can just about guarantee you'll size down. Unfortunately, many mums never end up sized, keep buying 24mm shield kits thinking that's the 'regular' size, and never have a good experience. Hopefully this post will help.

After thousands and thousands of sizing conversations with mums via DM and email, we can tell you with very high confidence: 99% of pumping mums need a smaller shield. Once mums are properly sized, the two most common sizes we end up recommending are 16mm and 18mm.

Most birth professionals get this wrong too. Midwives, lactation consultants, hospital staff — a lot of them learned on older guidance and haven't updated. So if no one's told you any of this — that's why.

This guide walks through the signs your shield is wrong, why snug is the right feel, how to measure yourself, the full size range we stock, and what sizing actually is and isn't.

Signs your shield isn't the right fit

There are a few specific signs. None of them are subtle once you know to look.

Your areola is being pulled into the shield. This is the biggest sign. A correctly fitted shield holds the nipple inside the funnel — the areola should stay outside, against the breast. If you can watch the pump run and see your areola being drawn into the shield, that's a too-big shield, no further diagnosis needed.

It hurts. Real pain — during or after pumping. Persistent tenderness between sessions, cracking, redness, splitting. Often one breast worse than the other.

Your nipple swells much larger than it should. Nipples will always swell to the 'funnel' part of the shield they're drawn down into — a bigger shield means more swelling. A too-big shield forces your nipple to swell past what its anatomy wants, which can block the milk ducts and reduce output. This is why "snug" matters — it's not about comfort, it's about not forcing the nipple to swell into a state that blocks the flow.

A few other things to watch for. Poor output from the start — not a "sudden drop", but a baseline that was always lower than expected. Flow that stalls mid-session (starts strong, stops). Asymmetry between breasts, where one side is consistently doing worse than the other.

One thing to know: it's very unlikely your nipple size changes suddenly. So if you've been pumping fine and your output crashes overnight, it's more likely something else than a sizing change. Wrong fit usually shows up as a baseline problem — pain and poor output from day one, not an out-of-the-blue drop.

It's almost never your supply.

Your shield should be snug

A snug shield isn't a problem — it's how a correctly fitted shield should feel. It holds your nipple inside the funnel without much room to spare. That's what keeps your areola where it belongs and stops your nipple swelling past what your anatomy wants.

A little lubricant (we like olive oil) is your friend.

A few drops on the inside of the shield before you pump keeps everything comfortable. Sterile coconut oil works too. Most commercial lubricants either break the seal, aren't safe with breastmilk, or can even damage the sensitive skin of the nipples when used as a lubricant for pumping (rather than to moisturise - lanolin!) so stick to olive or coconut.

Don't size up because the shield feels snug. Size down because of the things we listed above — areola pull-in, pain, over-swelling.

How to actually measure yourself

Here's the method we walk through with every mum who DMs us. The measurement is one input — what's happening inside the shield while you pump is the other.

Step one: get a nipple ruler. Google "shield size nipple ruler" — there are loads of free printable templates. They're printed sheets with circles cut at every millimetre. Print one on regular paper, no special supplies needed.

Step two: pump for a couple of minutes, then measure. Just a couple of minutes — enough to draw your nipple out. Then put your nipple through the ruler and find the smallest hole it fits through. That's the size to try. A full session can over-swell your nipple if your current shield is too big, which gives you a falsely large reading.

Step three: send a video too. A ruler photo alone isn't enough. What we really need to see is what's happening inside the shield while you pump — is your areola being drawn in? Is your nipple over-swelling? A short clip of one pumping session, plus the ruler photo, gives us everything we need to recommend a size.

Step four: don't measure too large. Most mums do. When your nipple's at the edge of two sizes, the smallest hole that fits is the answer — even if it looks tight.

Try the 24mm first. Then get sized.

Every Spectra pump ships with a 24mm shield in the kit. As we said up top — that's a packaging reality, not a recommendation.

The order we'd suggest:

1. Start with the 24mm that came in your kit. Pump with it for a few sessions. Get a sense of what pumping feels like.
2. Then size down based on how it feels and your nipple ruler measurement.
3. Expect sizing to be part of your journey. This is definitely something to expect to spend some time on and try a few things with - think about how hard it is to get the right shoe fit. Your nipples even more so!

The full Spectra size range

We stock every size you're likely to need in our breast shield collection.

• Wide neck shields: 16, 20, 24 and 28 mm.
• Standard neck shields: 20, 24 and 28 mm.
• 14 mm wide neck is coming in approximately August.
• Handsfree cups, only available in 24mm and 28mm unfortunately, but do a google for 'shield inserts' and you'll find plenty of third party options. These pop into the shield to create the right fit. Using the right size is super important on cups! Without a good fit, you'll be unlikely to achieve a good seal and suction will be poor.

Most current Spectra pumps use wide neck. If you've got an older Spectra or a specific compatibility need for standard, check what your pump uses before ordering replacements — wide and standard aren't interchangeable.

If you've worked out you need a 14 mm and you can't quite wait until August, DM us — we can help you with an interim option.

Sizing isn't a perfect science

Shield sizing isn't a perfect science. Sometimes the best shield for you isn't the one that's the "correct" fit — it's the one that feels best for you and gives you the best results.

Elasticity, flat nipples, and factors unique to each mum mean shield sizing is always individual. You should go with what feels best.

Two things to remember:

This is something to experiment with. Try a size. Pump for a few sessions. See how your breasts respond. If output drops or pain starts, try a different size.

Snug is generally the best guidance. If you're still not sure, lean towards the smaller snugger fit.

Not sure? Send us a photo and a video

We do sizing consultations every day. If you're not sure what size you need, here's what to send:

1. A photo of your nipple through the nipple ruler, taken right after a pump session. Show the smallest hole it fits through.
2. A short video of one of your pumping sessions, showing the nipple inside the shield as the pump runs. We're looking for what the nipple is doing — touching the walls, being pulled into the funnel.

DM us at @spectrababy on Instagram or email through our contact form. We'll work out the right size with you.

You don't have to figure this out on your own.

~ Spectra Team 🩷