How does Marijuana effect future children? How is it worse than mothers who smoke cigs or drink alcohol?
From what I recall of studies that I read a LONG time ago, is that use of marijuana causes future children to lack ambition and initiative. The studies did not address why--they probably couldn't, since the tools for doing in-depth genetic analyses weren't available 2 or 3 decades ago.
As I recall, the effect was not from marijuana use during pregnancy, but marijuana use preceding pregnancy (even by years). That suggests to me that the DNA in the egg cells was chemically altered in such a way as to change gene expression, which, in turn, affects fetal development.
In the current literature, there are a number of studies that detail the harm of marijuana use during pregnancy. It apparently damages the brain and immune system of the fetus; it also apparently causes an increase in a certain kind of childhood cancer. These effects are all mediated through altered gene expression (caused by chemical alteration of the DNA, which does not change the DNA sequence, but affects how genes are expressed).
Cigarette smoking during pregnancy harms the fetus by restricting the flow of oxygenated blood through the placenta. Limited alcohol use during pregnancy doesn't seem to do much, but heavy use affects fetal development, probably through cytotoxic effects of the alcohol on highly sensitive rapidly growing cells.