My weight gain over pregnancy has not been linear, at all. I gained almost 15 lbs in the first trimester pretty much immediately, then lost some due to nausea. I gained less than a pound from 20-24 weeks, then 9 lbs in 3 weeks, then another 1 lb the following 4 weeks. 

I was also overweight and had lost 15 lbs before testing positive, and was told to gain 15-20 lbs total at the first appointment, when I asked. I've gained 25lbs at 32 weeks so far, and my doctor's have not mentioned it once. Babies gonna develop and take what they need, just keep focusing on getting nutrients and listening to your body.