Baby clothing sizes are usually based on age: preemie, 0 to 3 months (newborn), 3 to 6 months, 6 to 9 months, 9 to 12 months, 12 months, 18 months, and 24 months.

Newborn wardrobe essentials: Onesies: 3-4; Sleepers/stretch suits: 3-6; ... Age doesn't mean much when it comes to baby clothes; different brands size clothing differently.