Maternity Size Calculator: Trimester-Adjusted
This maternity size calculator adjusts by trimester — most charts treat pregnancy as one phase, when bust, waist, and belly grow at different rates across nine months. Enter your pre-pregnancy size and current trimester; the tool returns the right maternity size so you don't size up unnecessarily. Works for tops, dresses, jeans, and nursing wear. Use the calculator below to get your exact fit in 30 seconds.
How it works
Measure yourself
Take your bust, waist, and hips with a soft tape. See the full measurement guide.
Enter the numbers
Type your measurements into the calculator above. Pick inches or cm — the tool converts both ways.
Get your size
Match the result against the brand's chart and order with confidence.
Maternity sizing by trimester
| Trimester | Bust | Belly | Hip |
|---|---|---|---|
| Pre / Q1 | Pre-pregnancy | Pre-pregnancy | Pre-pregnancy |
| Q2 (14–27w) | +1" | +3–5" | +1" |
| Q3 (28–40w) | +2" | +8–12" | +2–3" |
| Postpartum | +1–2" | +2–4" | +1–2" |
Frequently Asked Questions
Should I size up for maternity clothes?
Order your pre-pregnancy size in maternity cut. Maternity panels and ruching are built into the garment — sizing up makes the chest/shoulder fit wrong.
When do I switch from regular to maternity sizes?
Most women switch at 14–18 weeks (second trimester). Stretch maternity panels accommodate growth from there to delivery.
Does maternity sizing differ between brands?
Yes — some brands cut maternity tighter (J.Crew Maternity), others looser (Old Navy, Pinkblush). Always check the trimester guidance on the product page.
What about postpartum and nursing sizing?
Match your post-delivery size, not pre-pregnancy. Most women settle at +1 size for 3–6 months postpartum. Nursing tops have crossover panels that stretch with you.
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