Maternity Clothes Sizes: How to Choose and When You Need Them
Maternity clothes are graded to grow with the bump, so the rule is simpler than many expect: buy the size you wore before pregnancy. This page covers the size chart, how the bump grows week by week, which waistband suits each stage, and what changes with bras and postpartum clothes.
Buy maternity clothes in your pre-pregnancy size: if you wore a US 8 (UK 12, EU 40) or a medium, you wear maternity 8 or M. Maternity patterns already add room for the bump, bust and hips. Most people start needing them in the second trimester, roughly weeks 12-20, when the uterus rises out of the pelvis and reaches the navel by about week 20.
- Size to buy
- Pre-pregnancy size
- Bump at the navel
- about week 20
- Typical gain (BMI 18.5-24.9)
- 25-35 lb (11-16 kg)
- Fundal height
- cm ≈ weeks (20-36)
On this page
Maternity size chart
Maternity labels use the same numbers and letters as regular womenswear. The size you choose is based on your measurements before pregnancy; the garment then supplies the extra room.
| Maternity size | Pre-pregnancy US | UK | EU | Pre-pregnancy bust | Pre-pregnancy hip |
|---|---|---|---|---|---|
| XS | 0-2 | 4-6 | 32-34 | 32.5-33.5 in (83-85 cm) | 34.5-35.5 in (88-90 cm) |
| S | 4-6 | 8-10 | 36-38 | 34.5-35.5 in (88-90 cm) | 36.5-37.5 in (93-95 cm) |
| M | 8-10 | 12-14 | 40-42 | 36.5-37.5 in (93-95 cm) | 38.5-39.5 in (98-100 cm) |
| L | 12-14 | 16-18 | 44-46 | 39-40.5 in (99-103 cm) | 41-42.5 in (104-108 cm) |
| XL | 16-18 | 20-22 | 48-50 | 42-44 in (107-112 cm) | 44-46 in (112-117 cm) |
| 1X-3X | 14W-24W | 18-28 | 46-56 | Plus maternity follows the brand's women's chart | |
If you do not remember your size, measure your hip now: hips change less than the waist in early pregnancy and usually point to the right size. Full non-maternity sizes are on the dress size chart.
How the bump grows, week by week
Clinicians track the top of the uterus (the fundus) against landmarks on the body. This is also a practical guide to when waistbands stop working.
| Week | Top of the uterus | Clothing stage |
|---|---|---|
| Up to 12 | Inside the pelvis | Regular clothes; bloating may tighten waistbands |
| 12-16 | Just above the pubic bone | Unbuttoned jeans with a belly band, stretch waists |
| About 20 | At the navel | Maternity bottoms; longer tops needed |
| 20-36 | Rises about 1 cm a week; fundal height in cm roughly equals weeks | Over-bump panels, maternity dresses, bigger bras |
| About 36 | Near the bottom of the ribcage | Maximum size; may drop slightly before birth |
Fundal height is measured from the pubic bone to the top of the uterus. At 28 weeks it is typically about 28 cm (11 in), give or take 2 cm. Your total belly circumference depends on your build, so maternity brands design around the pre-pregnancy size rather than a bump number.
Recommended weight gain and what it means for sizing
Most of the weight gained in pregnancy is the baby, placenta, fluid, blood and the uterus, not fat on the hips or arms. That is why most people stay in their usual size of maternity wear throughout. The ranges below are the 2009 Institute of Medicine guidelines, endorsed by ACOG, for a single baby.
| Pre-pregnancy BMI | Total gain | Total gain (kg) |
|---|---|---|
| Under 18.5 | 28-40 lb | 13-18 kg |
| 18.5-24.9 | 25-35 lb | 11-16 kg |
| 25-29.9 | 15-25 lb | 7-11 kg |
| 30 and over | 11-20 lb | 5-9 kg |
Your own target is something to agree with your doctor or midwife. For clothes, the practical point is that gaining within or near these ranges rarely needs a size change in maternity wear, while gaining well above them often shows at the hips, thighs and upper arms, which is where to size up.
Panels and waistbands: what fits each trimester
| Type | How it fits | Best stage |
|---|---|---|
| Belly band | Stretch tube worn over unbuttoned regular pants | Weeks 10-20, and postpartum |
| Side panels / adjustable elastic | Inset elastic at the hips with button tabs | Late first to second trimester |
| Under-bump | Low, curved stretch band beneath the belly | Second trimester onward; postpartum |
| Over-bump / full panel | Tall stretch panel covering the whole bump | Second and third trimester; most support |
Leggings and jeans are the two items worth buying in a maternity cut early; both rely on a fitted waist that regular sizing cannot fix. The leggings size chart covers inseams, and the pant size chart explains rise. Tops need extra front length more than extra width: a regular top that fits the bust will ride up over the bump by mid-pregnancy.
Bras during pregnancy and nursing
The bust often changes first, sometimes from the first few weeks. The ribcage widens to make room for the baby, so the band size can increase as well as the cup.
- Remeasure each trimester. Many people need at least one band size and one or more cup sizes larger by the third trimester.
- Buy stretch and adjustability: soft cups, a band with several hook positions, and wide straps. Underwires that press on growing breast tissue are uncomfortable for many.
- Nursing bras are usually bought from about 36 weeks, when the size is closest to what you will need after birth, with room for the milk coming in. The nursing bra size guide explains how to fit them; general band and cup measurement is in the bra size guide.
Measuring yourself during pregnancy
- Bust: around the fullest part, in a non-padded bra.
- Under-bust: directly under the breasts, snug. This is your band measurement.
- Bump: around the widest part of the belly, usually at or just below the navel. Some maternity charts give this figure by size.
- Hip: around the fullest part of the seat.
Compare these with a maternity brand's chart only if it lists pregnancy measurements; otherwise stay with your pre-pregnancy size. If you are carrying twins, expect the bump to reach third-trimester size several weeks earlier, and prefer over-bump panels and adjustable styles.
Wrap dresses and wrap tops, stretch jersey dresses, adjustable-strap overalls and button-front shirts in a size up cover most of pregnancy and nursing. A wrap adjusts at the waist, and button fronts give nursing access later.
Frequently asked questions
Should I size up in maternity clothes?
No, not as a rule. Maternity sizes are cut for a pregnant body in your pre-pregnancy size, with extra room at the bump, bust and hips built in. Size up only if you have gained more than about 20 lb (9 kg) outside the bump, are carrying twins, or are between sizes in a fitted woven style.
When should I start wearing maternity clothes?
Most people switch between about 12 and 20 weeks. In a first pregnancy the bump often shows later, nearer 16-20 weeks; in later pregnancies, or with twins, earlier. Bloating can make waistbands tight before the bump itself shows.
What is the difference between over-bump and under-bump?
Over-bump bottoms have a tall stretch panel that covers the whole belly; under-bump bottoms have a low, curved waistband that sits beneath it. Over-bump suits the third trimester and those who want support; under-bump is cooler and works for low-carrying bumps and postpartum.
Can I wear regular clothes in a bigger size instead?
For loose tops and dresses in stretch fabric, often yes. For pants and fitted tops it rarely works: a larger regular size grows everywhere, so it bags at the legs and shoulders while the front still rides up, because it lacks the extra front length a bump needs.
How long will I wear maternity clothes after birth?
Many people keep wearing maternity bottoms for several weeks after birth. The uterus takes about six weeks to return to its pre-pregnancy size, and the waist and hips often take longer. Under-bump styles and nursing tops are the most useful postpartum.