I actually lost 10lbs in my first trimester. From the start of my second until now at almost 34w I've gained a total of 25lbs. That's including the 10 I lost. So technically 15lbs gained from second trimester-34w.
It varies for everyone, but I read somewhere that the "ideal" weight gain for the first trimester is about 3 lbs if you started at an average weight. I didn't really start gaining weight until the middle of my second, but my mother-in-law gained 20 pounds by the end of her first trimester.