I weigh once per week and track my weight on an app and it plots it against an "expected minimum and maximum". I did this in my first pregnancy as for some reason I found it fascinating (I am a little strange!) Some weeks I could put on 3-4 pounds in just one week without changing my diet, whereas other times my weight didn't budge!
I put on 2 and a half stone, weighed the day after I got back from hospital (so 3 days after birth) and I'd lost nearly 2 stone of that! The rest all except 3 pounds fell off without me dieting over the next few weeks.
Don't worry if you've only gained a little bit, or feel that you've gained too much. Some people put on nothing until the third trimester, others put on stones right at the start and then their weight gain stops or they even lose weight towards the end of pregnancy. As long as baby is growing ok, don't worry! If you do gain weight, it can all come off again when baby is here.
Additionally, the whole "pound a week" weight gain isn't anything that you can really expect as everyone is different, every pregnancy is different. Sometimes is just down to genetics and there's nothing that you can do to stop weight gain! Also you'll have spurts of weight gain and other times not.