When Tillie was 2 months old she still wore newborn sized clothing. Around 7 months she seemed to catch up and now she wears the sizes based on her age.
Except for pants. I tried these 18-24m Old Navy jeans on her today. Since she's wearing a cloth diaper, they're a pretty good fit. Yes, the waist is a little big (no adjustable waist on these.) and they're just slightly too long. But they fit better than her too short 12-18m jeans! These haven't been washed yet and I expect they'll be even better after I wash and dry them.