Finding the right bra size can feel confusing at the best of times — and then pregnancy, breastfeeding, and postpartum changes come along and shake everything up again. Searching for sister sizing, maternity bra sizing, or how to find your nursing bra size is incredibly common during this stage of life. That’s where sister sizing comes in.
At Hotmilk, we’re passionate about comfort, support, and confidence through every stage of motherhood. Understanding sister sizing can help you find your best fit when your body changes, your size is temporarily out of stock, or you just want a little flexibility in how your bra feels.
What Is Sister Sizing?
Sister sizing means moving up or down in band size while adjusting the cup size accordingly, so the overall cup volume stays the same.
In simple terms:
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When the band goes up, the cup goes down
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When the band goes down, the cup goes up
These sizes are called “sisters” because they fit a similar breast volume, just on a different band.
Example:
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12E → 14DD → 10F
All three have a similar cup volume, but the band fit changes.
Why Sister Sizing Is So Helpful for Mums
When it comes to bra fitting during pregnancy and breastfeeding, flexibility is key.
Pregnancy and breastfeeding bodies are constantly evolving. Sister sizing gives you options when your usual size doesn’t feel quite right.
1. Your Ribcage Changes During Pregnancy
As your baby grows, your ribcage often expands. You may need a looser band for comfort — but you still want the same cup capacity.
✔ Try going up a band and down a cup.
2. Postpartum & Breastfeeding Fluctuations
Milk supply, engorgement, and feeding patterns can all affect how your bra feels from day to day.
✔ Sister sizing allows flexibility without sacrificing support.
3. Comfort Preferences Matter
Some mums prefer a firmer band for support, while others want a softer, more forgiving fit – especially for sleep or long days.
✔ Sister sizing lets you fine-tune comfort while keeping the right cup volume.
4. Your Size Is Out of Stock
If your perfect size isn’t available, sister sizing can often save the day.
✔ A sister size may give you a near-identical fit until your size is restocked.
How to Find Your Sister Sizes
This method is commonly used by professional fitters when helping mums find the correct maternity bra size or best nursing bra fit.
Start with your usual size, then use this guide:
If the band feels too tight:
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Go up one band size
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Go down one cup size
If the band feels too loose:
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Go down one band size
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Go up one cup size
Example:
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Wearing a 14D but the band feels loose? → Try a 12DD
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Wearing a 12F but the band feels tight? → Try a 14E
Important: Sister Sizing Isn’t a Perfect Swap
While sister sizing is incredibly helpful, it’s not a forever solution for everyone.
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Band support changes as you move sizes
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Wire-free and structured bras may feel different in sister sizes
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Breast shape and tissue distribution still matter
That’s why we always recommend: ✔ Trying bras on if possible ✔ Choosing comfort first ✔ Re-measuring regularly during pregnancy and breastfeeding
Sister Sizing & Hotmilk Bras
Hotmilk specialises in maternity bras, nursing bras, breastfeeding lingerie, and now lingerie specifically designed for post-breastfeeding – Evolve; all created to adapt with your changing body.
Our maternity and nursing bras are designed to support your body through change — with flexible fits, forgiving fabrics, and wire-free options that work beautifully with sister sizing.
If you’re unsure, our fit experts are always here to help you:
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Find your ideal size
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Explore sister size options
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Choose the best style for your stage of motherhood
The Takeaway
Whether you’re searching for sister sizing explained, how to size a maternity bra, or the best bra for breastfeeding, understanding sister sizing can make all the difference.
Your body isn’t static — and your bra size doesn’t have to be either.
Sister sizing is a simple, empowering way to adapt to change, stay comfortable, and feel confident in your lingerie at every stage.
Because at Hotmilk, we believe you should never have to compromise on comfort, support, or feeling like you.
Need help finding your sister size? Reach out to our team or explore our fit guides online.
