Sister sizing for a nursing bra: when your size falls right between two

You bought it a size up and the band still digs into your back. You take it off in the evening, see the red line and think: this nursing bra is just no good. Fair enough that you'd think that. Except there's probably nothing wrong with this bra at all. You corrected one sizing axis the wrong way.

That's the pattern almost every mother hits when she falls between two sizes. You can feel something isn't right, you reach for a bigger size, and then one of two things happens. Either the band still squeezes, because you went bigger in everything. Or the bra gives you no support and your breast slips out the side, because you only pushed the cup letter up. Two different mistakes, the same wrong conclusion: it's the bra, or worse, it's me.

Sister sizing is the logic that sorts that out. And right after birth that logic runs a little differently from what the average size guide online tells you. That's why this article exists.

Short answer. Cup volume is set by your band size and your cup letter together, not by the letter on its own. An 80H and a 75I hold roughly the same amount of breast, the band just sits differently. So: if the band pinches, go one band size up and one cup letter down at the same time (from 80H to 85G). If the band is too loose, go one band size down and one cup letter up at the same time (from 80H to 75I). That way you shift the size around without losing your cup volume. If, on top of a poor fit, you also feel feverish, or notice a red, warm breast or a hard spot that won't clear, it's no longer a sizing question but a reason to call your GP or a lactation consultant.

What sister sizing actually is

A bra size is made of two numbers working together: the band size (the figure, 75, 80, 85) and the cup (the letter). The letter doesn't tell you how big your breast is in absolute terms. It tells you how much bigger your breast is than your band. A G cup on a narrow band is a smaller cup than that same G on a wide band.

The whole idea follows from that. Shift the band by one step and the cup volume shifts with it, unless you let the letter move too. To keep the same volume and change only the size around your body, you turn two dials at once. And this is exactly where everyone goes wrong, so let's be sharp about it. The rule of thumb is: band bigger, letter smaller. Band smaller, letter bigger. So:

• Band too tight, you want more room: band one step up, cup letter one step down. 80G becomes 85F.
• Band too loose, you want it snugger: band one step down, cup letter one step up. 80G becomes 75H.

Those three sizes (75H, 80G, 85F) are called sister sizes. They hold roughly the same amount of breast. Only the band sits tighter or looser. That's the dial you turn when a bra feels right in the cup but not in the band, or the other way round.

The two wrong guesses (and how to spot yours)

Almost nobody knows that rule of thumb, so almost everyone corrects on the wrong axis. There are two versions, and the symptom tells you straight away which one is yours.

Guess 1: just bigger in everything. You grab the largest size the brand makes, or you go up a size or two across the whole label. It feels logical: more bra for more breast. The result is a band that cuts into your ribs. One mother on Reddit put it bitterly: it's so tight on my ribcage that I can't wait to take it off, and that's with the biggest size they do. Another bought an XL that dug into her ribs while her breasts, by her own account, weren't even that big. What went wrong: you raised the band when only the cup needed to go up. The cup may have needed a little extra volume, but you handed your ribcage another 5 to 10 centimetres of circumference it never asked for.

Guess 2: only the cup letter up, band the same. You keep your familiar band size and move only the letter. Now the opposite happens: no support, or a band that goes too loose and rides up at the back. Your breast slips out the side of the cup the moment you lie on your side. On the forums you read the despair that follows: either the cups never get big enough, or there's zero support, or it's irritating within an hour, and then the question of whether this is just a universal nursing-bra problem or whether she's asking too much.

That last bit is the heart of the emotional trap. A sizing-axis mistake doesn't feel like a sizing-axis mistake. It feels like am I asking too much, or like nursing bras are just no good. And the moment you think that, the search stops. When you were one step away from right.

Why sister sizing goes extra wrong after birth

The standard sister-size explanation assumes you have one fixed starting size. You know you're an 80G, the band pinches, so you move to 85F. Done. But postpartum, nothing stands still, and that's exactly why the usual intuition breaks here.

Your band and your cup move independently after birth, and at different speeds. Mothers' own stories show it plainly. Someone at 32 weeks pregnant is wearing an 80H cup when she's normally a 75F: the band went from 75 to 80 and the cup from F to H, both at once. Another mother describes her whole run: a nursing bra in a 90 just before the birth, a 90I at the peak of feeding, and back down to an 85H a few weeks later. The band shrank while the cup first kept growing. Try to guess one static size in that phase and you're almost bound to get it wrong.

And it doesn't only drift over weeks. It drifts within a single day. One mother noticed that even around her period she was two cup sizes bigger in the morning than in the evening. Your breast fills and empties all day, every day. A cup that feels perfect just after a feed is too small three hours later. That's exactly why this is the phase where most mothers conclude the bra is no good, when the bra fitted fine for half the day.

What that means for sister sizing: don't treat it here as a fixed formula but as a fitting trick. You're not calculating your one correct sister size. You're using the sister sizes to shift around until both band and cup are right at the moment you're at your fullest. How to measure yourself at that moment is set out step by step in how to measure your nursing bra size in this phase.

Why S, M, L and XL hide the problem

Plenty of nursing bras aren't sold in number sizes but in S, M, L and XL. That sounds easy and is sometimes genuinely handy for this phase, because a stretchy size soaks up the daily swing. What it does hide, though, is the band-to-cup ratio this whole article is about. XL tells you nothing about your band and nothing about your cup. It only says: bigger than L.

Mothers with a bigger cup run straight into that. Someone who's always been a cup F, now bigger, puts herself around a G and sighs that plenty of bras go up to XL but nowhere does it say clearly what size that actually is. That's pure guesswork. An XL that runs bigger mainly in the band gives you guess 1 exactly: a band cutting into your ribs while the cup is still too snug.

Two things help when you're buying by letters instead of numbers:

1. Look up the size chart in centimetres, not just the S/M/L table. A good chart translates the letter back into a band and cup range. If it isn't there, you're buying blind.
2. Judge the band and the cup separately when you try it on. Band cuts in but the cup fits? You were in a too-small band translation of that label. Roomy band but too little support? The other way round.

Brand differences make the guess shakier still

Even when you know your size, the same size runs differently from brand to brand. One mother bought a 90G and found the cup came up a touch smaller than her usual size. That's not a mistake of hers and not a mistake of the brand, it's a known fact of bra land: seams, fabric and cut all vary. For you it means a size on the label is a starting point, not a guarantee. Trust what you feel when you try it on, not the figure on the tag. And try it on at your fullest, never just after a feed.

How to apply sister sizing without guessing

Here's the sister-size table you need. Find your starting size, see which way you need to go, and shift one row across.

Read it like this. If the cup fits but the band pinches, move to the column to the right of your starting size: band one step roomier, letter one step smaller, same cup volume. If the band is too loose, go to the column on the left: band tighter, letter bigger. If the band fits but the cup doesn't, sister sizing isn't your answer. Then you stay on your band size and adjust only the letter, simply because your cup really is too small or too big.

The practical postpartum version in four steps:

1. Measure and try on at your fullest, so just before a feed or right after you get up. What feels good straight after a feed tells you nothing about the rest of your day.
2. Choose your band at the moment it feels tightest (in the morning, with full breasts) and let it sit just comfortably there. Starting on the loosest hook gives you room for the fuller days.
3. Let the cup take the volume. In this phase your breast swells by the hour. A cup with some stretch isn't second best here, it solves exactly this: you don't have to hit the exact size.
4. Measure again after a few weeks. Your size in week two isn't your size in month four.

When pinching is no longer a sizing question

A bra that leaves welts or a band that cuts in, you swap. Usually that's the whole story. But sustained pressure on a breast full of milk isn't harmless. The milk ducts sit just under your skin, with little tissue around them to protect them. La Leche League describes how, in the spots where a bra pinches, you can develop a blocked duct. The Academy of Breastfeeding Medicine explains in its mastitis protocol (ABM #36, 2022) that the tissue around the duct then swells, the duct narrows and the milk moves through it less easily. Left to carry on, that can turn into mastitis. Your bra is rarely the only cause, but it's the factor you can take away today.

Get in touch with your GP or midwife, or in the daytime a lactation consultant or health visitor, if you have:

• a fever or a fluey, run-down feeling alongside a breast complaint;
• a red, warm, painful patch in your breast, especially if you feel yourself getting rapidly more unwell. This can point to mastitis, an inflamed breast;
• a hard spot or lump that doesn't clear after a feed or after a few days;
• increasing pain, pus or blood;
• bloody or unusual discharge from one nipple, separate from cracked nipples.

For breastfeeding support in the UK you can turn to your health visitor or a local infant feeding team, or a lactation consultant (IBCLC). But with a fever or a red, painful breast, your GP is your first port of call, not a support line; out of hours, contact your out-of-hours GP service or NHS 111. There's more on the line between normal and a warning sign in the signs that a nursing bra pinches or fits wrong.

From giving up to one step away from right

The end point of the sizing-axis mistake is resignation. The mothers' own words run straight to it: one woman can barely find ordinary bras in her size and has given up on nursing bras altogether. Another wonders out loud whether a bra that came up a bit fuller is enough for a new cup size, the exact between-two-sizes moment. And on the forums you hear the weary recognition: never found a decent one, what a saga.

That resignation keeps the mistake alive, because it puts the blame in the wrong place. Not I guessed the band-cup combination wrong, but nursing bras are no good, or I'm asking too much. When the opposite is true. The problem was small and fixable: one axis, one step. Not you, not your figure.

And there's the case, right there, for a bra that soaks up the daily swing instead of punishing it. The less you depend on the exact size, the less there is to guess. If you'd first like to know the best moment to buy your first nursing bra, read when to buy a nursing bra: pregnancy or the early days.

Where our nursing bra fits into this

Everything above lands on one point: a fixed size on a breast that changes volume all day. That's what we set out to solve, with stretch instead of a rigid number.

Our nursing bra is wireless and knitted from a fabric that gives as your breast fills up towards feeding time, so the band keeps supporting without cutting in at your fullest, and the cup takes the volume without you having to hit the exact size. The straps are wide and the clip opens one-handed. The absorption sits in the cup itself, four layers that catch up to 30 ml per cup (60 ml per bra). The fabric is OEKO-TEX STANDARD 100, Class I certified (certificate number 11-36224), the strictest category: for textiles that may touch a baby's skin. Designed in Belgium, knitted in Türkiye.

You can take a look here: see the nursing bra. And if you'd first like everything about size, fit and wearing comfort side by side, head to the overview of nursing bras, fit and comfort.

Frequently asked questions

What exactly does sister sizing mean for a bra?

Sister sizes are sizes with the same cup volume but a different band. The rule: go up a band size and the cup letter goes down a step, and the other way round. That's how 75H, 80G and 85F all hold roughly the same amount of breast. You use it to change the size around your body without losing your cup volume.

I took the biggest size and the band still pinches. Now what?

Then you probably corrected on the wrong axis. You went bigger in everything, when only your cup needed to go up. Go one band size roomier and one cup letter smaller at the same time: from 80G to 85F. The cup holds the same amount, but the band no longer cuts in. Always try it on at your fullest, just before a feed.

How can my cup feel too small while the band is too loose?

That's completely normal after birth, because your band and your cup change independently. Often your ribcage narrows again while your breasts are still full. Go one band size tighter and one cup letter bigger at the same time: from 80G to 75H. The band supports again and the cup still fits.

Does sister sizing work with S, M, L and XL too?

Not directly, because those letters hide the very band-to-cup ratio sister sizing works on. XL tells you nothing about your band or your cup. Look up the size chart in centimetres and judge the band and the cup separately when you try it on. For the daily volume swing, a stretchy size is handy.

What time of day should I measure myself?

At your fullest, so just before a feed or right after you get up. Your breast fills and empties all day, and some mothers are two cup sizes bigger in the morning than in the evening. Measure just after a feed and you'll buy a bra that pinches the rest of the day. Measure again after a few weeks.

Read on in this guide

• Which nursing bra is genuinely leak-proof? Absorbent options compared
• The best nursing bra for each moment: night, work, sport and a bigger cup
• Why are my breasts leaking? Causes and what's normal
• Night-time leaking: what to wear in bed
• Breast pads: which do you need and when?