Smoking is harmful to the human body. Marijuana smoke is just as dangerous as cigarette smoke. However, THC is not harmful to mother, or to baby in small ammts. I don't think it is a good idea to be getting "high" while pregnant, but I know plenty of women who vaporized or used marijuana teas and edibles to combat morning sickness. I do not smoke pot, but I did use an edible once during my current pregnancy to help out with especially severe M/S. It worked very well. I just didn't like the buzzy/spacey feeling it gave me.
Re: marijuana
BFP1: DD1 born April 2011 at 34w1d via unplanned c/s due to HELLP, DVT 1 week PP
BFP3: DD2 born Feb 2013 at 38w4d via unplanned RCS due to uterine dehiscence