That kind of weight gain is not out of the realm of possibility. Once they start nearing the 1000 gram mark, if they are consistent feeders, they can easily put on 200 grams per month. I typically find that they put on 100-150 grams per month after 1000 grams, but have had some do the 200 grams per month spurts (sometimes even for 2 or 3 months in a row).