Hi Dear, Welcome to HCM. Understanding your concern. As per your query you have marijuana addiction and yes smoking marijuana during pregnancy could affect foetus in many ways and it could affect since first trimester till last trimester. It could result in low birth weight babies, miscarriage, abortion. It can cause growth problem in foetus as well. If after baby birth you will start having it again it will reach by milk and will stay in baby system and will start giving adverse effects. It will lower down immunity of your body as well as that of incoming foetus and susceptibility to certain infectious diseases like asthma increases. I would suggest you to stop it immediately. If it is giving you much addiction taper the dose and eventually stop. Hope your concern has been resolved. Get Well Soon. Best Wishes, Dr. Harry Maheshwari