For the uninitiated, baby clothing sizes are v. confusing. Allow me to explain so you don't accidentally try to bring baby home from the hospital in an adorable 0-3M layette.
American Baby Clothing Sizes
American sizes measured in “months” generally fit babies in an age range counting down from that number to the next available size. For example, something sized 3-6M generally will be a bit large around three months and a bit snug around six months of age. Also, size 3-6M is identical to the size 6M.
American sizes measured in “years” will generally fit kids who are currently that age. For example, items in size 2T will generally fit a child that is two years old, starting off a bit big and ending up a bit snug about a year later. Same goes for items sized as 2-3Y.
You will almost certainly experience some size overlap, with your baby fitting into as many as three “sizes” at the same time. That’s because every brand fits a little differently and has slightly different sizing guidelines based on weight and height, not age (despite almost all brands using age as their measurement of choice). To know for sure whether or not an item in a specific size will fit your kid, you’ll have to consult the brand’s size chart, which is usually available on their website.
American Baby and Toddler Sizes
Preemie (P) – Generally fits babies weight under 5-7lbs. Babies who weigh under 5lbs fit into a special even smaller size, sometimes called “Micro Preemie” or “Preemie Under 5lbs.” You will likely not need this size and do not need to buy any clothes in preemie sizes unless you happen to give birth to a preemie.
Newborn (NB) – Generally fits babies weighing under 9lbs, but some brands run especially small and only fit babies under 7lbs. If you give birth to a 9+ pounder, it’s possible that you’ll skip the newborn size altogether. You’ll get more bang for your buck by choosing stretchy fabrics and not buying too many items in this size in the first place.
0-3M / 3M – Generally fits babies weighing 7-12lbs. Your typical 7.5lb newborn will swim in most 0-3m clothing, but it will start to fit around week five or six.
3-6M / 6M – Generally fits babies weighing 12-17lbs.
6-9M / 9M – Generally fits babies weighing 17-20lbs. This is a smaller, snugger size than 6-12M or 12M.
9-12M – Generally fits babies weighing 19-22lbs.
6-12M / 12M – Supposedly fits babies weighing 17-22lbs, but actually fits babies on the larger side of that range.
12-18M / 18M – Generally fits babies weighing 22-27lbs.
18-24M / 24M – Generally fits toddlers weighing 27-30lbs. This is a smaller size than 2T.
2T / 2-3Y – Generally fits toddlers weighing 30-33lbs.
3T / 3-4 Y – Generally fits kids weighing 33-36lbs.
4T / 4-5Y – Generally fits kids weighing
European Baby Clothing Sizes
European sizes are measured in centimeters, which makes sense since the sizing relies more on the length of your baby than his or her age. Generally a brand will just pick a number, rather than a range. For example, Hanna Andersson's clothes run in sizes 50, 60, 70, 75, 80, 85, and 90. You can assume that the 50 will fit a newborn, whereas the 60 will fit a baby in the 0-3 months range. Not all European brands use the same round numbers, and instead have sizing that reflects the length of their garments.
European Baby and Toddler Sizes
< 50 = Preemie – Generally fits babies weight under 5-7lbs. Babies who weigh under 5lbs fit into a special even smaller size, sometimes called “Micro Preemie” or “Preemie Under 5lbs.” You will likely not need this size and do not need to buy any clothes in preemie sizes unless you happen to give birth to a preemie.
50-56 = Newborn (NB) – Generally fits babies weight under 9lbs, but some brands run especially small and only fit babies under 7lbs. If you give birth to a 9+ pounder, it’s possible that you’ll skip the newborn size altogether. You’ll get more bang for your buck by choosing stretchy fabrics and not buying too many items in this size in the first place.
56-62 = 0-3M / 3M – Generally fits babies weighing 7-12lbs. Your typical 7.5lb newborn will swim in most 56-62 clothing, but it will start to fit around week five or six.
62-68 = 3-6M / 6M – Generally fits babies weighing 12-17lbs.
68-74 = 6-9M / 9M – Generally fits babies weighing 17-20lbs. This is a smaller, snugger size than 6-12M or 12M.
74-80 = 9-12M – Generally fits babies weighing 19-22lbs.
80-86 = 12-18M / 18M – Generally fits babies weighing 22-27lbs.
86-92 = 18-24M / 24M – Generally fits toddlers weighing 27-30lbs. This is a smaller size than 2T.
92-98 = 2T / 2-3Y – Generally fits toddlers weighing 30-33lbs.
98-104 = 3T / 3-4 Y – Generally fits kids weighing 33-36lbs.
Hope this helps!