Maternity clothes sizing: buy your usual size, at the right time
Maternity sizes follow your pre-pregnancy size: the room for your bump is built into the cut. The real question is when to buy what.
Updated October 13, 2026 · sources checked October 13, 2026
In short
Most maternity clothes are cut to your pre-pregnancy size, with extra room in the bump, bust and hips built in, so start from the size you usually wear. Most weight is gained after week 20, says the NHS, so buy stretchy basics early and bump-specific pieces in the second trimester. Measure bust, under-bust, waist and hips each time you shop, as they keep changing.
How maternity sizing works
Maternity clothes are labelled with ordinary sizes. A maternity size 10 (UK) or 6 (US) is designed for someone who wore that size before pregnancy: the extra room for your bump, a fuller bust and wider hips is built into the cut, the stretch of the fabric and the adjustable waistbands. Mothercare's size guide puts it in one line: if you were a size 8 before pregnancy, you will fit a size 8 maternity garment, and a medium stays a medium. Sizes still vary slightly between brands, so check each brand's chart against your own measurements.
How much your body changes, and when
Your clothes have to follow your weight gain, and that gain is not even across the nine months.
| Source | What it says |
|---|---|
| NHS | Most pregnant women gain 10 to 12.5 kg (22 to 28 lb), with most of it put on after week 20 |
| MedlinePlus (US) | Most gain 2 to 4 lb (1 to 2 kg) in the first trimester, then about 1 lb (0.5 kg) a week for the rest of pregnancy |
| NIDDK (NIH) | 1 to 4 lb in total in the first 3 months, then 2 to 4 lb each month from month 4 until delivery |
The US recommendations depend on your BMI before pregnancy:
| BMI before pregnancy | Recommended total gain (NIDDK) |
|---|---|
| Below 18.5 | 28 to 40 lb |
| 18.5 to 24.9 | 25 to 35 lb |
| 25 to 29.9 | 15 to 25 lb |
| 30 or more | 11 to 20 lb |
These are health guidelines from your care team's point of view, not clothing targets, and every body carries a pregnancy differently. A meta-analysis of over 1.3 million pregnancies found that gaining above or below these ranges was linked with different risks for mother and baby (Goldstein et al., 2017); your midwife or OB is the person to discuss your own weight with. For shopping, the useful fact is simpler: the first trimester changes little, the second and third change a lot.
How to measure yourself
Use a soft tape, stand straight, wear a thin top or your bra, and breathe normally.
- Bust: around the fullest part of your chest, tape level all the way round.
- Under-bust: snugly around your ribcage, just under your breasts (for bras).
- Waist or bump: around your waist at its fullest point, which later in pregnancy is your bump.
- Lower hip: around the fullest point, roughly 20 cm (8 in) below your waist, as Mothercare's guide describes.
- Write the date down: measure again before each new purchase.
Compare these numbers with the brand's chart. If you are between two sizes, the stretch of the fabric and the adjustability of the waistband should decide.
A buying plan by trimester
| Stage | What usually changes | What to buy |
|---|---|---|
| First trimester | Little visible change; waistbands may already feel tight; bust may change | Belly band or stretch waistband to extend your own trousers; a soft, non-wired bra if yours feel uncomfortable |
| Second trimester | Bump becomes visible; most weight gain starts after week 20 | Leggings or trousers with an over- or under-bump band; 3 to 5 stretch tops; one dress; a supportive bra fitted to your current size |
| Third trimester | Bump at its largest; swelling of feet common late in the day | Longer tops and dresses that cover the bump; comfortable low-heeled shoes; nursing bras and nursing tops for the end of pregnancy and after |
Waistbands: over-bump or under-bump?
- Over-bump (a wide panel over the belly): stays up well and smooths the line under tops; can feel warm.
- Under-bump (a band below the belly): cooler, closer to ordinary trousers; can slip as the bump grows.
ACOG notes that some maternity trousers come with a wide elastic band under the curve of the belly that helps support its weight, a useful detail if your back aches. If you need more support than a waistband gives, see our maternity belt guide.
Shoes
Feet can swell late in the day. ACOG recommends low-heeled (not flat) shoes with good arch support for back comfort; the NHS suggests comfortable shoes with no tight straps when feet swell. Try shoes on in the afternoon.
What you really need, and what is nice to have
| Really useful | Nice to have |
|---|---|
| 2 to 3 pairs of maternity leggings or trousers | Maternity jeans for weekends |
| 3 to 5 stretch tops, some with nursing openings | A going-out dress |
| A supportive, comfortable bra in your current size | Maternity pyjamas |
| Comfortable low-heeled shoes | A coat or layer that closes over the bump (essential if you are due in winter) |
Printable checklist: maternity wardrobe
Spending wisely
- Buy pieces that work beyond pregnancy: wrap dresses, stretch tops with nursing openings, leggings with a fold-down band.
- Wait before buying third-trimester sizes: measure again around week 28.
- Choose breathable fabrics (cotton, cotton-rich jersey, viscose blends) and check the care label.
- A nursing bra fitted late in pregnancy can serve you from the third trimester into the feeding months.
Is this normal? When to call
Outgrowing your clothes fast, swollen feet at the end of the day and a bra that suddenly feels tight are all ordinary parts of pregnancy. What needs a call is sudden change, described below. For urgent advice in the UK, NHS 111 or your maternity unit; in the US, your OB or midwife.
Sudden swelling of your face, hands or feet, a very bad headache or vision problems such as blurring or flashing lights need a call to your midwife, GP or labour ward straight away (NHS: possible pre-eclampsia). In an emergency, call 911 in the US or 999 in the UK.
Questions parents ask
Should I size up in maternity clothes?
Usually not. Maternity ranges are designed around your pre-pregnancy size and add the room for the bump themselves. Size up only if a brand's chart, measured against your current bust and hips, says so.
When should I start wearing maternity clothes?
When your usual clothes stop being comfortable, which for many women is in the second trimester. The NHS notes most pregnancy weight is gained after week 20, so that is when bump-specific pieces earn their keep.
What maternity clothes do I really need?
A few pairs of stretch leggings or trousers with an over- or under-bump band, several stretchy tops, one comfortable dress, supportive bras and a coat or layer that closes over the bump if you are pregnant in winter.
Can I wear maternity clothes after the birth?
Yes, and many women do for the first weeks. Choose tops and dresses with nursing openings if you plan to breastfeed, so they keep working after the birth.
Sources
- NHS — Weight gain in pregnancy checked October 13, 2026
- NIDDK (NIH) — Health Tips for Pregnant Women (weight gain by BMI) checked October 13, 2026
- MedlinePlus — Managing your weight gain during pregnancy checked October 13, 2026
- ACOG — Back Pain During Pregnancy (support waistbands, shoes) checked October 13, 2026
- NHS — Swollen ankles, feet and fingers in pregnancy checked October 13, 2026
- Mothercare — Maternity size guide (how maternity sizes work, where to measure) checked October 13, 2026
- Goldstein RF et al. (2017), JAMA — Gestational weight gain and maternal and infant outcomes, meta-analysis (PMID 28586887) checked October 13, 2026
Mamavela explains; it does not replace your midwife, doctor or pediatrician, who know your situation. If something worries you, call them. In an emergency, call 911 (US), 999 (UK) or 112.