My mini one's 15months-ish now and is in 6-9 months with plenty of room! she's teeny tiny! K was 2 in nocember and is in a right mish-mash of sizes! waist wise, she's in 12-18m, but length wise she needs 2-3 really! but cos they fall off her waist they look too long!
but for the mummies of bigger babies - K never wore newborn, she went straight into 0-3 and by 3 weeks she was in 3-6m! she just seemed to lose all her chubbyness!