weight

So, I lost about 8 pounds when I became a vegitarian over the last few months. I was a very healthy weight (BMI 22.2) before the vegitarian thing and now im close to under weight (BMI 20.2). now I am almost 8 weeks pregnant and have gained close to 3 pounds. should I count my weight after my 8 pound loss as my prepregnancy weight and only gain the 4 for the first trimester. or should I count my prevegitarian weight as my prepregnancy weight to stay healthy?