Weight loss & pregnancy

I am 13 weeks pregnant, and weigh 161. I was 149 prior to getting pregnant, and gaining weight like crazy. I was on topamax, which restricted my eating habits a lot before getting pregnant, and I think me not being on it is contributing a lot to my over-indulging/steady weight gain. I am so conscious about it, and don’t want to continue gaining unnecessary weight. How do I go about losing weight, if that’s even possible, and only gaining what I need to. This is my 4th pregnancy too.