As more states legalize medical marijuana and recreational marijuana, questions about the drug’s effects on a fetus will become a more central public health concern. One significant flaw with most studies looking at marijuana use in pregnancy is that researchers do not take into account that a large proportion of marijuana users also smoke tobacco. Since negative effects from smoking during pregnancy are well-established, it could be that these outcomes are the ones being detected when researchers study marijuana use in pregnancy without adjusting for the effects of smoking.
The findings of a new study would seem at first to allay any concerns about that flaw: after adjusting for tobacco use, a combined analysis of more than two dozen studies found no increased risk of low birth weight, preterm birth or a handful of other poor outcomes among newborns whose mothers used marijuana during pregnancy.
But a closer look reveals the same problems that have plagued previous attempts to investigate all the research to date: there simply are not enough high-quality studies that provide enough data on enough pregnancies to separate out all the possible effects that could interfere with identifying effects from marijuana.
“When the authors go from the adjusted analyses to the further adjusted analyses, they have to dramatically reduce their sample size,” noted Aaron Caughey, MD, chair of the Department of Obstetrics and Gynecology at Oregon Health & Science University in Portland. “Thus, the negative findings of no increased risk for preterm birth or low birthweight could just be because of inadequate study power.”
It’s also important to note up front that this study, published in Obstetrics & Gynecology, focused only on a handful of possible risks at birth and only at birth. “We did not investigate long-term neurodevelopmental outcomes after exposure to marijuana in utero, and further study is warranted in this regard,” the authors wrote. In fact, few studies exist at all that look at long-term effects of marijuana use during pregnancy, and of the handful that do, which Emily Willingham and I analyzed for our book’s section on marijuana use, almost none take into account the use of tobacco, alcohol or other drugs, making it hard to know what effects from marijuana, if any, exist.
Source : http://www.forbes.com/sites/tarahaelle/2016/09/09/so-does-marijuana-use-in-pregnancy-hurt-a-baby-or-not/#526594fa4065