I crocheted this dress using the Sweet Little Sundress pattern on a blog called Hyphenated and Overextended. I of course tweaked it a bit, changing up some of the stitches in the skirt and adding a fun shell stitch border to the bottom.
This dress (as the pattern is written) fits a newborn pretty well. The blog says a "5-7lb" baby in the comments, but I would put it closer to an 8-10lb baby, or even a 0-3 months if your stitching has a bit of stretch to it.
This pattern isn't difficult, but might be challenging for a beginner. It depends on how well you read and follow patterns! Adding a decorative border to the bottom is simple enough, but keep in mind it adds length to the gown...mine was a little long on my daughter!