I have found Carters to be wonderful at sizing. When they say up to 12.5 lbs... that's really what they mean! I tend to have long babies that fill out nicely.
My girl was out of their size 3mo at 2 months of age. She's 14 lbs now and well on her way through size 6mo. I keep telling her gramma (DH mom) that she'll need sizes DOUBLE her age. i.e. If she's 3 mo old, the we need 6 mo for her... when she's a year, I would wager she'll be in 2T's.
Anyway, just wanted to let you know if you keep ok track of LO's weight, you'll know what to buy for them in the Carter's line.