Sizing up infant clothing

Here's a photo of my Lindsay when she was crawling (probably 9 months old or so). She's wearing an outfit that buttons at the bottom, which meant it didn't ride up and show her belly while she crawled.

My younger sister had a baby in May, and she recently mentioned in her baby’s blog that her baby (now 4 ½ months old) is already wearing 6-month-size outfits. I want to assure her that baby Ivy is perfectly normal.
I have four years of experience working in children’s clothing store and can’t resist sharing my knowledge.
So here is

My Infant Clothing Size Primer.
Children’s sizes work something like this: In infant sizes (3 months up to 24 months), babies generally wear the clothes size that is twice the child’s actual age. So, the average 4-month-old wears a 6- to 9-month-old outfit; the average 6-month-old wears a 12 month size; a 9-month-old wears an 18 month size and so on. Between 12 months and 18 months, most babies are wearing 24 month size clothes.
A 24 month size and a 2T are generally about the same size as each other.

However, the infant sizes and the toddler sizes (2T-4 or 5T) are usually different styles, allowing for the differences between a baby and a toddler.

Infant clothes are generally one-piece outfits rather than two-piece. The one-piece design keeps the clothes from riding up when the baby is being carried or is crawling on the floor. Around 12 months (of course, it’s a couple months earlier for some babies, and a few months later for others), babies start to walk. At that point, they are able to wear separates easier – a shirt and a pair of pants for example. You don’t have to worry about the clothes riding up, thus, clothes in the toddler sizes are usually two piece outfits rather than one piece.
So there, in a nutshell, are the basics of infant clothing sizes.