Newborn or 0-3 months: which size will my baby wear?
In shortOn the Carter's chart, size NB fits 6 to 9 lb (2.7 to 4 kg) and 18 to 21.5 in (46 to 55 cm), and 3M fits 9 to 12.5 lb (4 to 5.7 kg). By the WHO median curves, a typical girl wears NB for 26 days and a typical boy for 19 days, so we suggest making most of the first clothes 3M, with only a few NB pieces.
Likely size at birthNB · newborn18 to 21.5 in (46 to 55 cm) and 6 to 9 lb (2.7 to 4 kg)
Moves up to 3M · 0 to 3 months at 23 days, by the WHO median curve (average of girls and boys).
A bigger baby moves up earlier, at 9 days; a smaller baby moves up later, at a little over 1 month.
Every brand fits differently: when in doubt, compare your baby's length with the brand's chart. The chart here is the official Carter's chart; the window of use of each size comes from the WHO median curve, and a bigger and a smaller baby are the 85th and 15th percentiles.
For a premature or small baby, Carter's also has UP5LB: up to 5 lb (2.3 kg) and PRE: up to 6 lb (2.7 kg).
Cotokim's list counts how many pieces of each size to have, from your due date and your city's climate.
Build my list for my cityNB or 3M at a glance
| Size | Weight | Length | A typical girl wears it | A typical boy wears it |
|---|---|---|---|---|
| NB · newborn | 6 to 9 lb (2.7 to 4 kg) | 18 to 21.5 in (46 to 55 cm) | up to 26 days | up to 19 days |
| 3M · 0 to 3 months | 9 to 12.5 lb (4 to 5.7 kg) | 21.5 to 24 in (55 to 61 cm) | 26 days to 2.8 months | 19 days to 2.2 months |
The weight and the length come from the Carter's chart 1. The time in each size comes from the WHO median curves of weight 2 and length 3: the day a typical baby passes the top of the size.
How we calculate
- The size by length and weight. A baby wears the smallest size whose top length and top weight they have not passed. If length and weight point to different sizes, go with the bigger one, so the clothes are not tight.
- The age by the WHO curves. The median curve is a typical baby; a bigger baby (85th percentile) moves up earlier and a smaller one (15th percentile) later: a bigger girl leaves NB at 11 days, and a smaller girl at a month and a half 2 3.
- Pounds and inches from the label. In the tool, 9 lb is still NB: on the chart, 9 lb is the top of NB and the start of 3M 1, and the tool keeps the smaller size until your baby passes the top. The kilograms and centimeters next to them are conversions.
Sizes for a small or premature baby
Carter's also has sizes below NB: Preemie, up to 6 lb (2.7 kg), and up to 5 lb, up to 5 lb (2.3 kg) 1. Our list does not count them, because a typical newborn is born bigger, on the WHO median weight curve 2.
Frequently asked questions
Should I buy newborn size clothes?
A few. A typical baby wears NB for 26 days (girl) or 19 days (boy) on the WHO median curve 2, and a bigger baby may skip it. Our suggestion: make most of the first clothes 3M.
How long does 0-3 months last?
On the WHO median curve, a typical girl wears 3M 26 days to 2.8 months and a typical boy 19 days to 2.2 months 2. On the Carter's chart, bodysuits in 3M cover the same length as sleep bags in 0-3M 1, so we read the number on the label as the end of the age range.
Does every brand fit the same?
No. Each brand uses its own measurements: compare your baby's length with the brand's chart, and then the weight.
Source credits
Brands appear only for the charts of their own products (sizes, packs, TOG). Cotokim has no deal with any brand.
Written by the Cotokim teamChecked by the Cotokim team against the sources on this page, on October 14, 2026
Sources
- Carter's, Kids Clothing Size Chart. Read on October 14, 2026.
- World Health Organization, WHO Child Growth Standards: weight-for-age. Read on October 11, 2026.
- World Health Organization, WHO Child Growth Standards: length/height-for-age. Read on October 11, 2026.