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According to a new review study, women who smoke marijuana during pregnancy won’t necessarily increase the risk of preterm birth or other harmful birth results.

The researchers did originally discover a general connection between smoking marijuana during pregnancy and an increased risk of having a preterm or low-birth-weight baby. When they took another look and took into account whether the pregnant women also smoked tobacco in addition to marijuana, this increase in risk went away.

To break it down more clearly: the risk of having either a preterm birth or a baby with a low birth weight was mostly due to tobacco smoking, and marijuana use by itself was not connected to these specific results, the researchers said.

The reports “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. Comparatively, the lack of a connection between marijuana use and harmful birth results 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]

Other studies on marijuana use during while pregnant have had opposing results, with some exhibiting that marijuana increases the risk of damaging birth outcomes and others pointing out that there is no increase in risk. However, 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 examined information from 31 older studies that together studied more than 7,800 women who used marijuana while pregnant and more than 124,000 women who did not use marijuana during pregnancy. The researchers only admitted the studies that were developed in a way that allowed them to examine marijuana use separately from tobacco use. They also included some studies that used objective measures of marijuana use while pregnant — such as a positive urine test — in addition to studies that used self-reports of marijuana use.

They established that overall, women who smoked marijuana while pregnant 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.

Then when they looked at women who smoked only marijuana while pregnant 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 comparison, 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 crucial to take note of the fact that the study did not look at the long-term health of babies whose mothers smoked marijuana during pregnancy, so more research is necessary in order to fully 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.

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