What size should I buy?

When does baby weight start to slow down? I have oldnavy supercash and want to use it before it expires today. I want to buy summer clothes for a vacation trip when she turns 14 months but I am not sure what size to buy. If she's 6 1/2 month old and weighs 21 Lb what size should I buy when she's 14months old - 3T or 4T?