i have heard that water arobics is very good for you,
Eating small healthy meals throughout the day is better than three big ones, exercising in moderation (not going beyond your pregnant ability) and just watching what you are eating, you can indulge your cravings once in a while, but try not to go overboard.
I don't have an answer to you question. But I'm 5'7 and weight about 165 so if you find a way to keep weight gain to 25lbs please let me know.