Doodlebug

0000 newborn - 4 months

000 3-6 months

00 6-9 months

0 /XS 9-18 months

S  1 - 3 years

M  3 -6 years

L  5 - 8 years

XL 7-9 years

If you prefer a slimmer fit, go with the smaller size.  If you prefer a loser fit (ie for boys) go with the bigger size.

Doodlebug clothes are designed to fit over several sizes.