Parental marijuana use likely does not affect pre and post natal development if used in moderation. However, if a mother is regularly abusing marijuana it is likely to have a negative impact on the development of both the fetus and, eventually the child. Mothers should take care to not overindulge.
Based on the information I have read in the past, I do believe that Marijuana use by a pregnant Woman can negatively affect Pre and Post-natal development of the child. This is why I think it is important to avoid any and all drug use when a child is being developed in the mother.