A new “study of studies” in the journal Obstetrics & Gynecology drills down on the data behind claims that marijuana can cause adverse affects on pregnant women and their unborn children.

The verdict: Tobacco did it.

Researchers from Washington University conducted a detailed analysis of more than 30 studies to ascertain whether marijuana use can cause low birth weight or premature delivery.

Although lead researcher Dr. Shayna Conner and her colleagues initially discovered that women who smoked marijuana while pregnant did have an increased risk for such problems, they also found that once a mother’s tobacco use was entered into the equation the relationship vanished.

“We found that maternal marijuana use during pregnancy is not an independent risk factor for low birth weight or preterm delivery after adjusting for factors such as tobacco use,” the study authors wrote. “There also does not appear to be an increased risk for other adverse neonatal outcomes  … once we account for other influencing factors.”

They  concluded, “The increased risk for adverse neonatal outcomes reported in women using marijuana in pregnancy is likely the result of coexisting use of tobacco and other confounding factors and not attributable to marijuana use itself.”

Writing for Reason magazine’s Hit & Run blog, Jacob Sullum points out that the evidence blaming marijuana consumption on birth problems has never been as prevalent as the well-documented facts against alcohol and tobacco.

“One reason for the lack of clarity is that pregnant women who use marijuana are more likely to smoke tobacco, which is known to raise the risk of premature birth and low birth weight,” Sullum wrote. “Hence tobacco smoking could be the actual cause of the negative effects associated with marijuana use in some studies.”

Therefore, while it is always a good idea to take into consider the potential risk factors that come with indulging in various substances (food, drugs, etc) while caring for the overall health of an unborn child, Sullum says, “Pregnant women who use marijuana are no more reckless than those who drink an occasional glass of wine (which, unlike heavy drinking, is not associated with birth defects), and they are running less risk than they would if they smoked tobacco.

“Since the government does not automatically assume that pregnant drinkers and smokers are unfit parents,” he continued, “it makes no sense to treat pregnant cannabis consumers that way.”