In most of the western world it remains illegal to purchase or consume marijuana. Depending on what state you live in, if THC (the psycho-active ingredient in marijuana) is detected in your body, you may actually have your child taken from you. This isn’t always the case, but certainly isn’t a risk worth taking.
On the other hand, if you’re living in an area that has legalized medicinal marijuana, you may be prescribed some form of non-combusting marijuana to help cope with pregnancy-induced nausea. Again, this isn't always the case, but the THC in marijuana is a powerful anti-nausea agent.
Smoking marijuana releases carbon monoxide (poison) into you and your child's blood and is contraindicated by the majority of the medical community during pregnancy as a result. The long-term effects of prenatal exposure to THC are actively debated within the medical community, but there is general agreement that the act of smoking itself, is the most harmful to the fetus, not the THC.
If you’re a regular smoker, you should resolve to quit smoking now. If you have a psychological dependency on marijuana, seek support from friends, family and/or a therapist to discuss your issues and fight your addiction accordingly.