With both of my pregnancies I put on the bulk of my weight in the first trimester, mainly because I ate quite a lot to ease my m/s. I had put on 6-7kgs by the time I hit 13 weeks both times, and at almost 30 weeks now am sitting on 10kgs. I put on a total of 15kgs with Izzy and am aiming for that again this time around. I am 5"1' and my starting pre-pg weight both times was 65kgs. As long as you are doing some sort of exercise regularly - even just walking, plus eating well I wouldn't worry too much.