One of the most common questions new parents — and gift-buyers — ask is: what size should I buy? Baby clothing sizes can feel baffling, especially when a "newborn" babygrow fits for just a few weeks. This guide breaks it down month by month so you always know exactly what to look for.
Why Baby Clothing Sizes Are Confusing
Unlike adult clothing, baby sizes are based on age ranges rather than precise measurements — and every brand sizes slightly differently. A 0–3 month sleepsuit from one brand might fit a baby that another brand would put in "newborn." The safest approach is to use age ranges as a guide, then check the weight and height guidelines on the label.
The golden rule: babies grow fast. It's almost always better to size up than down.
Tiny Baby (Up to 7.5 lbs / 3.4 kg)
Tiny Baby sizing is designed for premature babies or very small newborns. Most full-term babies will skip this size entirely, but if your baby arrives early or is on the smaller side, these pieces are a lifesaver.
Key items to have in Tiny Baby: a couple of sleepsuits, a hat, and scratch mitts. You won't need much — babies grow out of this size quickly.
Browse: Tiny Baby Collection
Current favourites include the Newborn Baby Pink Hearts Soft Cotton Romper & Hat Set and the Green Floral Bunny Baby Sleepsuit — both soft, simple, and easy to get on and off.
Newborn – 3 Months (Up to 14 lbs / 6.4 kg)
This is the size most full-term babies wear from birth. It's the most-gifted size for a reason — but be aware that bigger babies may skip straight to 0–3 months. If you're buying a gift and don't know the baby's size, 0–3 months is the safest bet.
At this stage, practicality is everything. Look for:
- Envelope necklines (easier to pull over a wobbly head)
- Poppers at the bottom for quick nappy changes
- Soft, breathable fabrics like cotton
- Sets that include a hat and scratch mitts
Browse: Newborn – 3 Months Collection
We love the Fruit de Ma Passion Beige Ruffle Collar Knitted 3 Piece Set with Bonnet — a beautiful gift-worthy set — and the Rock A Bye Baby Unisex Borg Pramsuit & Bear Comforter Set, perfect for cooler days out.
3 – 6 Months (Up to 16.5 lbs / 7.5 kg)
By three months, most babies have found their rhythm — and so have their parents. Clothing at this stage can be a little more fun and expressive. Babies are more alert, starting to smile, and spending more time being held and shown off to the world.
This is a great size to buy ahead for, especially if you're gifting. A baby born in spring will be in 3–6 months by summer, so think about the season they'll be wearing it in.
Browse: 3–6 Months Collection
Standout picks include the Home Grown Baby Dusky Blue Organic Cotton Ribbed 3 Piece Outfit — a gorgeous gender-neutral option — and the Baby Girls Pink Bobble Knitted 2 Piece Set for something a little more special.
6 – 12 Months (Up to 21 lbs / 9.5 kg)
This is a wide size range that covers a lot of developmental milestones — sitting up, crawling, pulling to stand. Clothing needs to be comfortable and allow for movement. Look for stretchy fabrics, easy fastenings, and nothing too fussy around the knees (crawling babies go through clothes fast).
Many brands split this into 6–9 months and 9–12 months, so check the label carefully.
Browse: 6–12 Months Collection
We love the Baby Neutral Knitted Sheep Jumper & Leggings Set — cosy, cute, and works for any baby — and the Fruit de Ma Passion Lemon Knitted 3 Piece Baby Set for a cheerful, gift-ready option.
12 – 24 Months (Up to 28 lbs / 12.7 kg)
By one year, babies are becoming toddlers — curious, active, and increasingly opinionated about what they wear (or don't). Durability matters more now. Look for machine-washable fabrics, reinforced knees, and easy on/off designs for little ones who are learning to dress themselves.
This is also a great age for more expressive, fashion-forward pieces — dresses, rompers, and coordinated sets that make for adorable photos.
Browse: 12–24 Months Collection
Current favourites include the Baby Girls Dark Coral Floral Dress & Bloomers Set and the Baby Girls Some Bunny to Love Ruched Tie Bubble Romper — both perfect for special occasions or gifting.
Quick UK Size Reference Guide
| Size Label | Approximate Age | Approximate Weight |
|---|---|---|
| Tiny Baby | Premature / newborn | Up to 7.5 lbs / 3.4 kg |
| Newborn – 3 months | 0–3 months | Up to 14 lbs / 6.4 kg |
| 3–6 months | 3–6 months | Up to 16.5 lbs / 7.5 kg |
| 6–12 months | 6–12 months | Up to 21 lbs / 9.5 kg |
| 12–24 months | 12–24 months | Up to 28 lbs / 12.7 kg |
Tips for Buying Baby Clothes as a Gift
- Size up — when in doubt, go bigger. A slightly large outfit is far more useful than one they've already outgrown.
- Think seasonally — consider what season the baby will be in that size. A summer baby in 6–12 months will be wearing autumn/winter clothes.
- Check the fastenings — poppers and zips are much easier than buttons for tired parents.
- Go for sets — coordinated sets take the guesswork out of getting dressed and look great in photos.
Shop by Age at AdoraBaby
Browse our full range of baby clothing by age: Tiny Baby | Newborn – 3 Months | 3–6 Months | 6–12 Months | 12–24 Months
Or if you're shopping for a gift, explore our Baby Gift Sets and Layette Sets — beautifully presented and always the right choice.