Figures from the American College of Obstetrics and Gynecology show that between 2 and 5 percent of women claim to have used marijuana during pregnancy despite concerns that using pot while pregnant can harm the developing baby.

Evidences on the dangers of using marijuana during pregnancy, though, have been spotty, and findings of a new review and analysis of earlier studies did not show independent link between pot use of a mother during pregnancy and adverse birth events such as preterm birth.

While the review initially found an association between smoking marijuana during pregnancy and elevated risk of having preterm birth or a baby with low birth weight, the increase in risk was eliminated once the researchers considered if pregnant women also smoked tobacco in addition to smoking pot.

This means that marijuana use by itself may not be responsible for preterm birth or low birth weight. It is likely that these outcomes are due to tobacco smoking.

Earlier studies on use of weed during pregnancy have different results. Some studies showed that using the drug boosts risks of harmful birth outcomes, while others did not find increase in risk. Many of these studies, however, were limited since they did not consistently consider tobacco smoking or rely on possibly unreliable self-report of women on marijuana use during pregnancy.

The new study from the October issue of the journal Obstetrics & Gynecology looked at information from 31 earlier studies involving more than 7,800 women who used marijuana while pregnant and more than 124,000 women who did not use the drug during pregnancy. The studies that the researchers looked into were those that were designed in a way that would independently analyze marijuana use from use of tobacco.

Study researcher Shayna Conner, from Washington University School of Medicine in St. Louis, and colleagues found that overall, pregnant women who smoked weed had 43 percent increased risk of having a baby with low birth weight and a 32 percent increased risk to have preterm birth compared with pregnant women who did not smoke marijuana.

When the researchers looked at the women who only used marijuana during pregnancy and did not smoke tobacco, however, they did not find increased risk for preterm birth or low birth weight. Those who used both marijuana and tobacco, though, were found to have 85 percent increased risk of having preterm birth compared with those who did not use any of the two substances.

The researchers likewise found no increase in risk for miscarriage or having small baby in women who smoked marijuana during pregnancy.

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