Lots of great replies. I would only be concerned if there is a stall in growth (longer than the normal platau) where the baby does not increase in weight or height. If the weight and height is still increasing and the growth curve is still a nice upward curve (no matter what chart the doc uses), not platued for a long peorid of time or starting to look like a hill or hump, keep on nursing!!!!
My daughter is about to be 15 months old and is 17 1/2 lbs. and 27 1/2 inches long. She actually has stopped growing from 7 months onward (and has lost some weight recently) due to a surgical procedure she has had. The docs didn't become overly concerned about it until her 12 month check up, 5 months of no growth in weight and height. If you see a pattern like this, no growth for 5-6 months in weight or height, that's when a concern needs to be raised. They have suggested formula for her, but so far all the hypo-allerginic stuff, she has not tollerated. We are adding 1 tbsp of canola oil to all of her foods for the extra calories, still no growth (She's now eating 3 meals a day!!!!), there is something else going on.........KWIM?