Researchers found that smoking marijuana during pregnancy doesn’t increase the risk of adverse birth outcomes like preterm birth or the birth of a baby with low weight. However, the findings do not encourage or condone marijuana use when pregnant.

The review published Thursday in the Obstetrics & Gynecology journal analyzed data from 31 studies comprising a total of 7,800 women who used marijuana during pregnancy and over 124,000 women who did not use marijuana during pregnancy.

Researchers initially found a link between marijuana use during pregnancy and a higher risk of adverse birth outcomes. But on taking into account whether the women also smoked tobacco in addition to marijuana, researchers found that among those who didn’t smoke tobacco, the increased risk no longer existed.

The researchers came to the conclusion that the increased risk was a product of tobacco usage and not marijuana.

Initial results showed that women who smoked marijuana during pregnancy were 43 percent more likely to give birth to babies with low birth weight. Moreover, their chances of having a preterm baby was up by 32 percent in comparison to women who didn’t use marijuana.

However, when researchers analyzed data of women who used only marijuana and not tobacco during their pregnancy they found no increased risk. On the other hand, women who smoked both marijuana and tobacco during pregnancy were 85 percent more likely to have a preterm birth compared to women who didn’t use marijuana or tobacco.

There was also no increased risk for miscarriage or having a baby smaller than normal for its gestational age among women who used marijuana during pregnancy.

Researchers from the Washington University School of Medicine in St. Louis, Missouri, reportedly said that their findings “do not imply that marijuana use during pregnancy should be encouraged or condoned.” They added that attention must be paid in helping women avoid using tobacco and other substances during pregnancy, which are known to cause adverse birth outcomes.

The study, however, did not look into the long-term effects on the health of babies whose mothers used marijuana during pregnancy.

"Any foreign substance that doesn't directly benefit maternal or fetal health should be avoided,” the study’s co-author Shayna Conner reportedly said.