The first days with a newborn have a curious way of speeding up time: feeds blend into short naps, visits, perfectly reasonable doubts and a seemingly endless series of diaper changes. At that pace, choosing between Size 0 and Size 1 diapers may sound like a minor detail, until there are leaks, marks on the skin or soaked clothes at three in the morning.
The right choice is not an exact science, but it is not a shot in the dark either. With a few simple signs and a cautious buying strategy, you can get it right almost every time and adjust quickly when your baby moves on to a new stage.
Why size matters from the very first days
In practice, a diaper is a piece of technical clothing: it has to seal, absorb, respect the skin and allow movement. With newborns, the margin for error is small because their bodies are tiny and wees and poos are frequent, with stools runnier than you might imagine.
A diaper that is too small can squeeze at the waist and groin, leaving marks and creating pressure points where the skin is most delicate. A diaper that is too big, on the other hand, tends to leave gaps at the back and legs, and leaks appear at the worst possible moment.
And then there is the belly button: in the first days, the umbilical stump needs space and minimal friction.
Size 0 and Size 1: what they mean in practice
Brands vary, but there is a common logic. Size 0 (often called “newborn”) is designed for very small babies or for a snugger fit in the first days. Size 1 (“newborn” on some packs) usually covers a wider weight range and is, in many cases, the most common starting point.
The best guide is not just the weight on the label. It is the whole picture: weight, length, tummy size, chubbier or slimmer thighs, and the way the elastic sits without digging in.
The table below helps you compare without turning the choice into an equation.
| Criterion | Size 0 | Size 1 |
|---|---|---|
| Typical weight range (may vary by brand) | 1-2.5 kg (Dodot) or 1-3 kg (Bambo Nature) | 2-5 kg (Dodot, Moomin Baby) or 2-4 kg (Bambo Nature) |
| Fit at the waist | Snugger and lower | More flexible and higher |
| Belly button area | Often with a cut-out/lower front | May require folding the front down for some babies |
| Absorption per diaper | Suited to small volumes and frequent changes | Tends to cope better with slightly longer intervals |
| Who it usually works best for | Premature babies, small babies, very “snug” first days | Most full-term newborns |
| Typical risk if the wrong choice | Squeezing if the baby is near the upper limit | Leaks at the legs/back if the baby is small |
If your baby was born full-term at an average weight, Size 1 usually works, as long as the fit at the legs is well sealed. If your baby was born small, lost weight in the first 48 to 72 hours (which is common) or has a slimmer build, Size 0 can give more security at the start.
Signs the diaper is too small (and when it is too big)
There is a difference between “fitting well” and “squeezing”. A well-chosen diaper sits firmly but lets you slide a finger into the waistband without effort and without leaving the skin marked for long.
The most common signs appear repeatedly, change after change, not as a one-off.
- Deep red marks in the groin
- Waistband rolling inwards
- Tabs at their limit and the diaper still not covering well
- Localised irritation where the elastic touches
- Difficulty fastening without pulling hard
When the diaper is too big, the pattern is different: gaps and leaks. Here it is worth looking at the “map” of the leak, because it gives clues about where the fit is failing.
- Leaks at the back: the waist is low or loose, or the baby moves and opens a gap when lying down
- Leaks at the legs: the side frills were not pulled out, or the leg opening is too wide for the thigh
- Damp clothes but a barely full diaper: urine escaped at the side before it could be absorbed
- Leaking after a feed: runny stools call for a snugger fit at the lower back and legs
A single episode can happen because a diaper was put on badly. Several episodes in a row call for a change of size or model.
Weight is not everything: body shape, belly button and growth rate
Weight is the starting point because it is easy to measure, but it does not describe the body. Two babies of the same weight can have different waists, more or less chubby thighs and different lengths. This changes how the diaper “hugs” the legs and how the waist seals.
In the first days, the belly button deserves extra attention. If the diaper rubs against the umbilical stump, it can cause discomfort and moisture in an area that should be kept clean and dry. Some Size 0 diapers come with a front cut-out; with Size 1, folding the front down is sometimes enough, as long as this does not create a thick fold pressing on the skin.
Growth rate also affects the buying decision. Some babies jump a size in two weeks; others stay comfortable in the same size for longer. There is no “normal” that applies to everyone, and that is liberating: the goal is comfort and containment, not following a calendar.
A practical detail: after the birth, there are often many changes in the first 24 to 72 hours because of meconium and how often babies pass stools and urine. At this stage, the right diaper is the one that makes changes quick and cleaning less endless.
How to buy without waste: a simple strategy for the first weeks
Buying diapers before the birth gives peace of mind, but buying too many can mean unopened packs once your baby has already changed size. A balanced strategy is to have the essentials and leave room to adjust after the first few days.
It works well to think “test first, stock up later”. The secret is to avoid giant packs until you have clear signs that the size and model fit your baby's body well.
- Buy a small pack of Size 0 and one of Size 1 (or ask for samples, if available)
- Use the size that seals best in the first 48 hours, watching for marks and leaks
- Only then invest in a larger pack of the winning size, always keeping a plan B for the following week
- Reassess when the tabs start getting close to their limit or when new leaks appear for no apparent reason
This approach also reduces frustration. If a particular brand does not fit your baby's body well, you can change early without being left with unused stock.
Step-by-step fit: put on and check in 20 seconds
Even with the right size, how you put it on makes a difference. A well-fitted diaper looks “tidy”, with no twists and no random gaps.
Start by centring the diaper at the back and bringing the front up, checking that the waist is level. Then fasten the tabs symmetrically; when one sits much higher than the other, it is a sign of misalignment.
Finally, two simple gestures solve many leaks: run your finger along the groin to “release” the frills outwards, and check that the diaper follows the line of the leg without being tucked in. And if the belly button is sensitive, make sure the front edge does not rub.
A correct fit usually allows movement without the diaper shifting. If the diaper “slips down” after a few minutes, it is either too big, not fastened properly, or the model does not suit your baby's body shape.
Overnight diapers, skin sensitivity and other variables that change the choice
Not all diapers of the same size behave the same way. Some models are thinner, others bulkier; some prioritise softness, others absorption. In newborns, the skin can react to fragrance, lotions or certain materials, which may mean changing brand even when the size is right.
Night-time also changes the equation. If your baby sleeps for slightly longer stretches, they may benefit from a model with greater absorbency, but this should not come at the expense of fit. Often the solution is not to “go up a size”, but to choose a more absorbent model in the same size, keeping the seal at the legs and waist.
Then there is the question of clothing: very tight bodysuits can press on the diaper and reduce its effective absorbency, because they compress the core. If you notice leaks mainly when your baby is dressed, try a size up in clothing before blaming the diaper.
And when you are still unsure between Size 0 and Size 1? There is a practical rule: if Size 0 is leaving marks or fastening at its limit, move to Size 1. If Size 1 leaves gaps at the legs and leaks at the back, go down to Size 0 or try another brand in the same size. You can usually feel the right choice day to day: fewer leaks, fewer changes of clothes, less irritated skin, and a sense that everything stays in place effortlessly.
