Baby

Baby sizing counts in months: newborn, 3M, 6M, 9M, 12M, 18M and 24M, with some brands splitting the difference as 0-3M or 6-9M. Each label names the age the garment is meant to fit up to, not the age it starts at. A 6M piece is built for a baby on the way to six months rather than one who has just arrived there. Cuts allow for a diaper at the back. The fastenings do the real work. Shoulder seams fold open, snaps run down the inside leg, and cuffs fold over the hands on newborn sizes.

Growth explains why this band moves so fast. A baby usually gains around ten inches in the first year and doubles birth weight by about five months. A size lasts weeks rather than seasons. Weight is usually the better guide on the chart than length, since round babies fill a size early. Buying one step ahead is normal and costs nothing on a bodysuit.

Sizing ahead has one limit worth naming. A footed sleeper with the foot in the wrong place bunches under the arch, and loose sleepwear is a safety question rather than a comfort one. In the US, sleepwear above size 9 months and up to size 14 must either fit snugly or be flame resistant. Infant sleepwear sized 9 months and smaller is exempt if it also stays within set maximum lengths and is labeled with the age in months. That boundary sits inside this band, so a 12M pajama answers to a different rule than a 6M one.