There's nothing you can do to prevent yourself from gaining weight. If you don't wanna gain fast stay away from carbs and sugars, and sugary drinks. It's normal to gain weight during pregnancy.
start walking :) . I still work out and im 19 weeks. Ive gained weight but not a crazy a lot due to me working out still.
Oh you said while prego.. eat smaller portions.. an in between eat fruits.. rather fill up on emm..
Breast feed that will help with some weight loss..
Make sure that what you're eating is healthy and has lots of fiber in it. Also, stay active...walk a lot. Your doctor will let you know if they are concerned about your gain and let you know what you can do...