How to Measure for Maternity Clothes by Trimester (1–9 Months)
Maternity sizing isn't "size up." It's "buy your pre-pregnancy size in maternity-cut clothing." The cuts adjust for the bump; the number stays put. Here's how to measure each trimester and pick what to buy when.
You're 16 weeks pregnant. Your jeans don't button. Your bras stopped fitting two weeks ago. The first instinct is to buy regular clothes one size up — and within 2 weeks they're tight again. That's because pregnancy doesn't grow your body uniformly; it grows your bust, belly, and (eventually) hips, while leaving inseam, shoulder width, and arm length the same.
Maternity sizing is engineered for this. The single rule: buy your pre-pregnancy size in maternity cuts. The garment expands; the size number doesn't.
The Single Rule
Maternity sizing
Why this works: maternity garments have built-in expansion features (ruched panels, stretchy belly bands, drawstring waists, side-tie wraps, faux-wrap fronts) that flex with the bump. The base measurements — shoulder, sleeve, hip, inseam — match your pre-pregnancy size.
Step-by-Step: The Five Numbers You'll Track
- Pre-pregnancy bust, waist, hip. If you don't have them, take them now (or use the smallest measurements you remember). These set your maternity size.
- Current bust. Bust grows 1–2 cup sizes through pregnancy. Re-measure every 4–6 weeks. See our bust measurement guide.
- Belly circumference. Around the fullest point, parallel to floor. Track monthly.
- Hips. Around the fullest point. Re-check at 24 and 36 weeks. Hip ligaments loosen and the pelvis can widen up to 2".
- Underbust (band size). The ribcage expands over pregnancy, so band sizes go up too. Re-measure when buying bras.
What to Buy by Trimester
Trimester 1 (weeks 1–13)
Trimester 2 (weeks 14–27)
• Maternity jeans (under-the-bump or over-the-bump)
• Maternity leggings (full-panel for support)
• Maternity tops (longer hem, ruched sides, faux-wrap fronts)
• Maternity dresses (empire waist, wrap dress, A-line)
• 2–3 maternity/nursing bras (1 cup size and 1 band size up from pre-pregnancy starting point)
Trimester 3 (weeks 28–40+)
• Full-panel over-the-bump pants (best support)
• Longer-hem maternity tops (regular tops ride up at this stage)
• Re-check bra sizing — many women go up another cup size during trimester 3
• Comfortable wide-toe shoes (feet swell up to 0.5 size)
Postpartum (first 6 weeks)
• Maternity leggings and over-the-bump pants (yes, still — your belly takes weeks to retract)
• Nursing bras (cup size larger than third trimester for milk supply)
• Loose tops that allow nursing access
• Don't pack pre-pregnancy clothes for the hospital — bring loose, soft, comfortable clothes a size larger than pre-pregnancy
Pants: Under-the-Bump vs Over-the-Bump vs Side-Panel
| Style | How it sits | Best for |
|---|---|---|
| Under-the-bump | Below the belly, on the hips | Trimester 1–2; petite frames; warm climates |
| Side-panel | Half-panel on sides only | Transitional; trimester 2; less restrictive |
| Full-panel (over-the-bump) | Stretchy fabric covers entire belly | Trimester 3; active pregnancies; back support |
| Demi-panel | Goes halfway up the bump | Trimester 2; flexibility for varying belly size |
Most women own a mix. Under-the-bump for hot weather, full-panel for comfort and support, demi-panel as the workhorse mid-trimester.
Tops and Dresses That Work Through 9 Months
Bra Sizing Through Pregnancy
The single most-changed garment during pregnancy is the bra. Both band and cup change:
| Stage | Band change | Cup change |
|---|---|---|
| Pre-pregnancy | Baseline | Baseline |
| Trimester 1 (8–12 wk) | +1 size | +1 cup |
| Trimester 2 (20–24 wk) | +1 size | +1 cup |
| Trimester 3 (32–36 wk) | Baseline | +1 cup |
| Postpartum / nursing | Baseline (or +1) | +1 cup (milk-supply driven) |
| After weaning | Returns to baseline | Often 1 cup smaller than pre-pregnancy |
Total trajectory: pre-pregnancy 34B → 36C trimester 2 → 36D trimester 3 → 36DD nursing → 34A after weaning. Plan accordingly. See our bra size chart.
Common Mistakes
Sizing up in regular clothes
A pre-pregnancy size 8 buying regular clothes in size 12 gets garments with 12-sized hips, 12-sized shoulders, and 12-sized inseam — most of which haven't grown. The garments fit loose everywhere except the belly, where they're still tight.
Buying everything for trimester 3 in trimester 1
Belly size 36 weeks ≠ belly size 16 weeks. Buy in stages — trimester 2 essentials when you transition; trimester 3 specifics in early trimester 3.
Skipping the underbust measurement for bras
Ribcage expands during pregnancy. A bra that fit in band size 34 trimester 1 may need band size 36 trimester 2. Re-measure underbust when bra-shopping.
Buying maternity clothes too early
Maternity wardrobes are pricey. Buy a few transition pieces (one pair of maternity jeans, one belly band, one wrap top) at week 12–14, fill in only as needed.
FAQs
What size maternity clothes should I buy?
Your pre-pregnancy size, in maternity cut. The garment is engineered for the bump; the number stays the same.
When should I switch to maternity clothes?
Usually weeks 12–16. Start with bottoms.
What's under-the-bump vs over-the-bump?
Under-the-bump sits below the belly. Over-the-bump has a stretchy fabric panel covering the belly.
Will my regular bras still work?
First 6–8 weeks usually yes. By trimester 2, buy maternity/nursing bras 2 cup sizes and 1 band size larger.
How does my body change after pregnancy?
Hips often stay 1–2 inches wider permanently. Bust changes for nursing duration. Belly returns over 6–18 months.
Can I just buy bigger regular clothes?
Works for some tops and dresses. Doesn't work for pants — regular pants up-sized fit wrong on hip and inseam.
One Number, Re-measured Often
Maternity sizing is your pre-pregnancy size — that's settled. The variable measurements (bust, belly, hip) are re-checked monthly so the bras and base garments fit. Pre-pregnancy size + monthly re-measure = a wardrobe that actually fits each trimester.
For the underlying body measurements, see our bust guide, waist guide, hip guide, and underbust + bust for bras.
Selling maternity on Shopify? Show pre-pregnancy sizing clearly.
Tailor Size Guide adds pre-pregnancy size matching to every maternity listing — so customers buy their actual size, not a guess.