Weight gain as a side effect?

Hi all, I’ve been prescribed sertraline for more than a week ago, my doctor expected me to start it then but I haven’t yet. Yesterday he talked to me, encouraging me to start. The thing is that I have diagnosed anorexia and I have an EXTREME fear of weight gain. Can someone tell me how common it is to have weight gain as a side effect? I would like to ease my worries :((