I've done a bit of research on this for you. Long story short, scientists don't know what the effects might be but it's widely advised not to use marijuana during pregnancy.
"While using marijuana isn't believed to directly cause physical birth defects, it could make you go into labor early or even affect your child's brain development."
Another article I read on this subject said... "Doctors have linked marijuana use during pregnancy with increased risk for pregnancy complications." These conditions include low birth weight, premature birth, small head circumference, and even stillbirth.
If you ask me, stay as far away from any form of drug during your pregnancy as possible.