If you were overweight when you got pregnant you are basically not supposed to put anything on til 20 weeks. Then after that it's around 1/2 a kg per week at most - so a weight gain of 10kgs all in all. I was overweight prior to conception and didn't put anything on (lost some at first) until after 20 weeks, and so far have only put on 5.5kgs (30 weeks today, so spot on so far, basically).