I would highly recommend that you refrain from
smoking tobacco during pregnancy, the dangerous part about smoking cannabis is inhaling burning plant matter, not the cannabis. Not to contradict Dr. Julia, but the alleged "dangers" of cannabis have thus far been thoroughly debunked, there has been no recent unbiased studies to support the outlandish claims made by most orthodox doctors.
http://pediatrics.aappublications.org/content/100/1/79.abstract?