Maternity Size Calculator
Find the perfect wireless fit for pregnancy or nursing. Select your stage to get custom ribcage expansion adjustments.
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Maternity & Nursing Bra Fitting Guide
During pregnancy, your body undergoes significant hormonal shifts that affect both breast volume and ribcage circumference. On average, pregnant women experience an increase of 1 to 2 band sizes and 2 to 4 cup sizes between early gestation and established breastfeeding.
Our Maternity Bra Size Calculator provides trimester-by-trimester sizing estimates to help you stay comfortable, supported, and pressure-free throughout your pregnancy and nursing journey.
Early Expansion & Sensitivity
Hormonal surges increase blood flow to breast tissue, causing tenderness and an initial increase of 1 cup size. Soft, stretchy wire-free seamless bras are ideal during this phase.
Ribcage Expansion
As the fetus grows, the diaphragm elevates and the ribcage expands outward. You may need a larger band size (or a bra extender) even if cup volume stays relatively constant.
Milk Supply & Drop Clips
Breasts reach their peak volume in early postpartum weeks. Look for flexible nursing bras with one-handed drop-down clips and space for absorbent breast pads.
Why Wire-Free is Recommended
Midwives and healthcare professionals strongly advise wearing wire-free bras during pregnancy and breastfeeding.
Rigid underwires can press against expanding milk ducts, increasing the risk of clogged ducts and mastitis. Soft-cup maternity designs provide flexible contouring without pressure points.
Maternity Sizing Tips
- ✓ Buy on the tightest hook: Unlike everyday bras, fit new maternity bras on the tightest hook so you can loosen the band as your ribcage expands.
- ✓ Use Sister Sizing: If your band feels tight at week 24, try going up a band size and down a cup letter (e.g. 34DD → 36D).
- ✓ Allow Room for Pads: Ensure cups have slight stretch to accommodate nursing pads without squeezing breast tissue.
Maternity Sizing FAQs
How do I calculate my maternity bra size?+
Start by entering your pre-pregnancy or current UK size in our calculator above. On average, expect to drop down a band size and go up one cup size by the end of the first trimester, with further changes as you reach the third trimester. If your ribcage expands, look for sister sizes with larger bands and smaller cups.
When should I buy a maternity bra?+
Around weeks 12 to 16, most expectant mothers feel their normal bras becoming too tight. It is wise to purchase only one or two transition bras, as your sizing will likely continue to shift throughout pregnancy and into nursing.
What is the difference between a maternity bra and a nursing bra?+
Maternity bras focus on comfortable support during pregnancy (using stretchier materials and wire-free shapes). Nursing bras include all those features but add drop-down clasps or crossover designs for convenient nursing access.
Can I wear an underwire bra during pregnancy?+
UK fitting experts and midwives generally suggest switching to wire-free options, especially after the first trimester. Underwires can compress developing tissue and block milk ducts. Stretchy, wire-free maternity styles offer flexible support and greater comfort.