Low weight gain

Hi, I am 29 weeks FTM here. I am 129 lbs and So far I have only gained 8 lbs. I am petite and the doctor is not much concerned about my weight. However I am a lot concerned. I am not starving myself or anything. Not sure why I dont gain weight. Has anyone been like me and given birth normally without issues. Any idea on what I can do to gain weight.