Most newborns fit into 0–3 month clothes from birth or within a few weeks, so that is the size to buy most of. Babies grow fast, and every brand sizes a little differently, so this guide shows the typical weight and height for each size, how many of each to buy, and how to tell when it’s time to size up.
Key points
- Baby sizes go by age, but your baby’s weight and length are a better guide.
- Buy mostly 0–3 months, plus a few 3–6 month pieces for when your baby shoots up.
- Newborn size only fits for a few weeks. Skip it, or buy just one or two if you’re expecting a small baby.
- If your baby is between sizes, go up a size for clothes. Sleeping bags are different: they must fit snugly at the neck and arms.
In this guide
Baby clothes size chart
These are typical UK sizes. Every brand cuts a little differently, so use them as a rough guide and go by your baby’s weight and length rather than their age.
| Size | Baby’s weight | Baby’s height | At Little Seeds |
|---|---|---|---|
| Newborn | Up to 4.5 kg (10 lb) | Up to 56 cm | Not stocked |
| 0–3 months | Up to 6 kg (13 lb) | Up to 62 cm | Bodysuits, sleepsuits, swaddles |
| 3–6 months | 6–8 kg (13–17 lb) | 62–68 cm | Bodysuits, sleepsuits, swaddles |
| 6–9 months | 8–9 kg (17–20 lb) | 68–74 cm | Bodysuits, sleepsuits |
| 9–12 months | 9–10 kg (20–22 lb) | 74–80 cm | Bodysuits, sleepsuits |
| 12–18 months | 10–11.5 kg (22–25 lb) | 80–86 cm | Selected sleepsuits |
Little Seeds sizes start at 0–3 months, and most of our printed sleepsuits, such as Stars and Nature, also come in 12–18 months.
Newborn or 0–3 months?
Newborn size fits babies up to about 4.5 kg (10 lb). The average baby born in the UK weighs around 3.4 kg (7.5 lb), and most babies put on weight quickly after the first week or two, so newborn clothes often only fit for a few weeks.
That’s why many parents skip newborn size and start with 0–3 months. The sleeves and legs may look a little long at first, but you can roll the cuffs, and many sleepsuits have fold-over scratch mitts that help. A bodysuit that’s slightly roomy is far more comfortable than one that’s too tight.
If your midwife has said your baby is likely to be small, pack one or two newborn-size outfits as well.
What sizes to buy before your baby arrives
It’s tempting to buy lots of tiny outfits, but you’ll get more use out of a smaller stack you can wash and rotate. A good starting point:
- 6 to 8 bodysuits in 0–3 months. A pack of 5 short sleeve bodysuits plus a few long sleeve ones covers most weeks.
- 6 to 8 sleepsuits in 0–3 months. Zip-up sleepsuits make night feeds and nappy changes much quicker.
- A few pieces in 3–6 months, so you’re not caught out by a growth spurt.
- One or two going-home outfits in newborn and 0–3 months for your hospital bag, since you won’t know your baby’s size until they arrive.
Think about the season too. A baby due in November will be in 3–6 month clothes by spring, so lighter layers will be more useful in that size.
Keep the tags and receipts on anything you haven’t used yet. It’s much easier to swap sizes once you know your baby’s weight.
Signs your baby needs the next size up
- The poppers at the bottom of a bodysuit are hard to close, or pull apart when your baby stretches.
- Their feet press against the end of a footed sleepsuit, or their toes are curled.
- The neck or cuffs leave red marks on their skin.
- The clothes ride up and leave their tummy or back showing.
- The sleepsuit pulls tight across their shoulders when you do up the zip.
Tight feet are the one to watch. Babies’ feet are soft and still growing, so move up a size as soon as a footed sleepsuit feels snug.
Sizing sleeping bags and swaddles
For clothes, it’s fine to size up. Sleeping bags are different. A baby sleeping bag must fit snugly around the neck and armholes so your baby can’t slip down inside it, so always choose the size that fits now, not one to grow into. Our sleeping bags come in 0–6 months and 6–18 months.
Our velcro swaddles come in 0–3 and 3–6 months. A swaddle should be snug around the chest but loose around the hips, so your baby’s legs can bend up and out. Read our swaddling guide for how to fit one safely.
Premature and small babies
Babies born early often need “tiny baby” or “early baby” sizes, which are smaller than newborn. If your baby is in a neonatal unit, ask the nurses what clothes they need, because some units ask for clothes that open fully at the front so they can reach monitors and wires. The charity Bliss has advice for parents of premature and sick babies.
Baby clothes size FAQs
Is 0–3 months too big for a newborn?
For most babies, no. 0–3 month clothes fit from around birth weight up to about 6 kg. They may look roomy for the first week or two, but they’ll fit for much longer than newborn size.
Do baby clothes sizes vary between brands?
Yes, quite a lot. One brand’s 0–3 months can fit like another’s 3–6 months. Check the brand’s size chart and go by your baby’s weight and length.
Should I size up when buying baby clothes?
For bodysuits and sleepsuits, going up a size when your baby is in between is fine. For sleeping bags, always buy the size that fits now.
How quickly will my baby outgrow each size?
Every baby is different. Many babies need the next size up after two to three months, but some move faster, especially in the early weeks. It’s normal to be in different sizes in different brands.
Do organic cotton clothes shrink?
Washing at 30°C and line drying, or tumble drying on low, helps any cotton keep its size and shape. Our guide to sensitive skin has tips on choosing a gentle detergent.
Little Seeds organic cotton bodysuits and sleepsuits come in sizes 0–3 to 12–18 months. Bodysuit packs and sleepsuits start at £9.99, with free UK delivery on orders over £35.
