DS is 10 months, 20 lbs and solidly in Carters 9-12 month clothes with some growing room (not tons, tho.) He needs more, though, and noticed ON is having some great deals on baby clothes. The thing is, their sizing is 6-12 months and 12-18. I can't figure out which size to get him. I want a size comparable to Carters 9-12. Thanks!