There are no studies on this using verifiable data, just as there are no studies using verifiable data about how many drinks it takes to harm the fetus. Because you just can’t do those kinds of tests. So all that’s known about it is what’s reported.
For instance, someone whose child has Fetal Alcohol Syndrome might report that she never had more than two drinks a day–when in fact it could be more. (She could be willfully lying, or she could just be unrealistic about how much she actually drank. Some alcoholics are totally in denial about how much they drink.)
I had four pregnancies in four different decades and the advice given ranged from “10 or fewer cigarettes a day will not hurt your baby” to “quitting will make your blood pressure go up” to “don’t even think about it, what kind of monster are you anyway?” With the result that my fourth child was never exposed to it in the womb.
(Pretty much the same with alcohol. “One or two drinks a day is fine, don’t get drunk” with the first, “Not one sip” with the last. Of course, with the first one I wasn’t old enough to drink, legally, anyway.)
I was told that side effects of smoking included low birth weight, premature delivery, and potential for miscarriage.
The results: In the '60s, smoking 10 cigarettes a day and 1 beer throughout the pregnancy
In the '70s, 10 cigarettes a day and cutting way back on previous alcohol consumption (which is to say, 2-3 drinks per week during pregnancy)
'80s, same as the '70s but I got more grief for it
'90s, no cigarettes, one drink late in term when barmaid at brunch got my virgin confused with my friend’s bloody. (Mary.)
From the '60s: 7 lb 15 oz, 4 days past due date, healthy
From the '70s: 7 lb 11 oz, 15 days past due date, healthy
From the '80s: 8 lbs, 10 days past due date, healthy (but skinny as an adult)
From the '90s: 7 lb 1 ounce, 3 weeks early, healthy
I don’t know what to say about pot. I would think it would result in a mellower baby but I don’t really know. I did have a friend who had to go off her epilepsy medication (I think it was Dilantin) during pregnancy but who, with her doctor’s knowledge, smoked a bit of pot which also worked to control seizures. This may have been a delusion of both my friend and her doctor–it was in the '70s, not that people can’t be deluded at any time.