I guess what's a normal weight gain depends on your BMI pre-pregnancy. If you were underweight to begin with, you'd be expected to gain a little bit more than the standard 25-35 lb weight gain overall.
I think I gained about 12 lbs at the end of my first trimester, which was a little more than I was expcting since I was just of the right weight for my height pre-pregnancy AND that despite the fact that I was throwing up pretty much everyday!
Right now, I've gained up 17 lbs total already, and while I wish it wasn't that much, I try not to worry too much about it. So long as the baby is healthy, and I try to eat as much healthy food as possible, it should be ok. I must admit, I do indulge in a chocolate here and there!
