Gerber baby clothes tend to run small. You could always pick up a few essentials at say Walmart in the 0-3 size and use those until the Carter's fit.
You'll find certain sizes run a bit larger - babyGap, Gymboree, H&M, Koala Baby, Amy Coe. Whereas Gerber, Babies R' Us, Ralph Lauren, baby Polo run small, IMO. Carter's and Crazy 8 seem to be in the middle.