Smoking marijuana during pregnancy doesn’t appear to increase the risk of preterm birth or other harmful birth outcomes, a new review study suggests.
The researchers did initially find a link between smoking marijuana during pregnancy and an increased risk of having a preterm or low-birth-weight baby. But when they took into account whether the pregnant women also smoked tobacco in addition to marijuana, this increase in risk went away.
In other words, the risk of having either a preterm birth or a baby with a low birth weight was due to tobacco smoking, and marijuana use by itself was not linked to these outcomes, the researchers said.
The findings “do not imply that marijuana use during pregnancy should be encouraged or condoned,” the researchers, from Washington University School of Medicine in St. Louis, wrote in the October issue of the journal Obstetrics & Gynecology. Rather, the lack of a link between marijuana use and harmful pregnancy outcomes suggests that attention should be focused on helping pregnant women to stop using tobacco or other substances known to have adverse effects on the pregnancy, they said. [7 Ways Marijuana May Affect the Brain]
Previous studies on marijuana use during pregnancy have had conflicting results, with some showing that the drug increases the risk of harmful birth outcomes and others showing no increase in risk. But many of these studies were limited because they did not consistently take into account tobacco smoking, or relied entirely on women’s self-reports of marijuana use (which can be unreliable).
In the new study, the researchers analyzed information from 31 previous studies that together included more than 7,800 women who used marijuana during pregnancy and more than 124,000 women who did not use marijuana during pregnancy. The researchers only included studies that were designed in a way that allowed them to analyze marijuana use separately from tobacco use. They also included some studies that used objective measures of marijuana use during pregnancy — such as a positive urine test — in addition to studies that used self-reports of marijuana use.