Hi,
    I understand your concern.
 Weed 
smoking has effect on development of fetal brain. You can try to nullify the bad effects/ increase healthy growth of your baby.
    Your answer lies in your query. You  say you dont want any thing bad to happen with your baby! maternal instinct is a great stimulator for doing positive things. So changing mind set up becomes easy.
    - Quit smoking weed immediately. Every time you have irresistible urge to smoke, think of your baby-& get indulged in some work of your interest.
    - Similarly, learn about nutrition & it's impact on fetal development. This will help you to eat well. Take a dietitian's help to plan your diet & stick to it.
     - B complex supplements do help in appetite. Use mouth watering soups before food, Have a well decorated dining table with colorful /appetite stimulating colors/ fragrance.
    - Add  natural antoxidents to your diet .
    - Get your 
thyroid level tested, take supplement if advised by 
endocrinologist.
  - Be regular in exercising/ sleeping/
meditation.
  All of these will help baby's mental/physical well being & also remove the bad effects taken place so far.
   Thanks