Baby clothing sizes are a moving target. One week a romper fits beautifully; a fortnight later you're wrestling the poppers closed and wondering whether you're imagining it. Because sizes are labelled by age but bodies grow at their own pace, the label almost never tells you when to move up — your baby does. Here are the actual signs a size has been outgrown, how to check, and what to do with the clothes they've left behind.
The Signs a Size Has Been Outgrown
Rather than guessing from the age on the tag, look for these:
- Crotch poppers strain or won't fasten. The single clearest signal on a bodysuit or romper. If you're pulling to close them, or they pop open during the day, the piece is done.
- The garment rides up. A bodysuit that creeps up the torso and untucks itself has run out of length — length is usually outgrown before width.
- Red marks on the skin. Elastic lines around the thighs, waist, or arms after undressing mean it's too tight, full stop.
- Toes curled at the end of a footed sleepsuit. Cramped feet are easy to miss and genuinely uncomfortable — check by feeling for where the toes end.
- Restricted movement. If your baby can't kick, stretch, or bring their arms up freely, the fit is limiting them.
- Shoulders and neckline pulling. A neckline that digs in, or an envelope neck that no longer stretches over the head easily.
One of these is enough. You don't need to wait for several.
Go by Weight and Height, Not the Age on the Label
Size labels are an average, and your baby is not an average. A large four-month-old may be in 6–9 months; a petite one may still be in 0–3. Neither is a problem — what matters is the fit. Most brands publish weight and height ranges alongside the age, and those are far more useful: measure or weigh your baby and shop against those numbers instead (our baby clothes size chart lays out the weight-to-size bands).
How Long Each Size Usually Lasts
Roughly, and with wide variation:
- Newborn: often just a few weeks — sometimes skipped entirely by bigger babies.
- 0–3 months: around 6–12 weeks.
- 3–6 months: around 3 months.
- 6–12 months: often longer, as growth slows.
- 12 months and up: sizes typically last several months each.
The practical lesson: the smallest sizes turn over fastest, so buy those in small quantities and top up the size your baby is actually in (see our layette checklist).
Should You Size Up Early?
Usually yes, within reason. A slightly roomy piece gets worn for months; a snug one gets worn for days. When you're between sizes, size up — with two caveats: sleepwear should fit properly rather than loosely for safety reasons, and a garment your baby is swimming in can bunch and restrict as much as one that's too small. Aim for room to grow, not room to get lost in.
What to Do With the Outgrown Size
- Sort straight away. Outgrown pieces should leave the drawer immediately — they only cost you space and time.
- Keep the quality pieces for a future sibling, washed and stored by size (see our storage guide).
- Pass on what's still good. Barely-worn baby clothes are the most welcome hand-me-down there is (see our hand-me-downs guide).
- Recycle the worn-out ones rather than passing on stained or thin pieces.
Stay a Step Ahead
The easiest way to avoid the scramble is to keep a small set of the next size up ready — not a full wardrobe, just enough to switch over the day the poppers stop fastening. Check the fit of the current size every couple of weeks in the early months, and you'll never be caught with a baby who's outgrown everything at once.
The Bottom Line
Move up a size when your baby tells you to — straining poppers, riding-up fabric, red marks, cramped toes, restricted movement — not when the age on the label says so. Shop by weight and height rather than age, size up when you're between, buy the smallest sizes sparingly, and clear outgrown pieces out of the drawer as you go. Check every couple of weeks and keep a few of the next size ready, and growth spurts stop being a problem.
Check our size chart before you order, and browse the next size up in our collections.