you can go by thw weight more.. alot of the baby clothes we have have weight sizes on them too.. usually 0-3 month goes up to 12lbs. Which is pretty accurate as colton is probably around 11lbs now and all of them still fit but some are to the point where I know in less than a month he will be wearing his 3-6 months (which is fine because it's starting to get warm and I bought those for the warm season

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Newborns said they go up to 8lbs, but Colton has just gotten to wear none of those fit (I guess they were alittle bigger), 0-3 month is like 8lbs-12lbs. I am not sure what the 3-6 months are, but I am going to guess somewhere around up to 16lbs. I would have to get some of his out and look.
Hope that helps!
And yeah, like Wobbles said.. with dresses (lucky girls.. lol) you can get away with it earlier because they are growing under them. You dont have to worry about them being too long or too short for awhile, unlike pants and stuff.