Can smoking marijuana cause a miscarriage?

No. There is no evidence that marijuana is associated with miscarriages. Marijuana has been used by millions in the US over the last several decades, including (undoubtedly) a lot of pregnant women. There is no evidence that it causes miscarriages, birth defects, or any other significant problem with babies.

There was also an extensive study in Jamaica of pregnant women who smoked marijuana heavily and found no evidence of any harmful side effects.