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.

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.

They found that overall, women who smoked marijuana during pregnancy were 43 percent more likely to have a low-birth-weight baby, and 32 percent more likely to have a preterm birth, than women who didn't smoke marijuana during pregnancy.

But when they looked at women who smoked only marijuana during pregnancy and did not use tobacco, they found that these women were not at increased risk for either preterm birth or low-birth-weight babies. In contrast, women who smoked marijuana and tobacco were 85 percent more likely to have a preterm birth, compared with women who didn't use either substance.

Women who smoked marijuana during pregnancy were also not at increased risk for miscarriage or having a baby that was smaller than normal for their gestational age.

It's important to note that the study did not look at the long-term health of babies whose mothers smoked marijuana during pregnancy, so more research is needed to look at this question. A 2014 study of animals and human cells suggested that marijuana use during pregnancy may affect the formation of connections between brain cells, possibility affecting brain development.

Original article on Live Science.

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