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Women who continue using marijuana up to 20 weeks' gestation have a five times greater increase in the risk of preterm birth, independent of other risk factors, the researchers report.
"Not only did continued use of marijuana increase risk for preterm birth, but it also made these births 5 weeks earlier, on average, with a greater number of women delivering very preterm," said senior researcher Claire Roberts, a professor at the University of Adelaide School of Pediatrics and Reproductive Health in Australia.
"That is much more dangerous for the baby, who inevitably would require admission to a neonatal intensive care unit," Roberts continued. "Earlier delivery would be expected to increase the baby's risk for dying and having long-term disabilities."
The more often an expecting mother used marijuana, the earlier her baby was born, the researchers also found.
The findings don't establish a direct cause-and-effect relationship. But the results suggest that more than 6 percent of preterm births could have been prevented if women did not use marijuana during pregnancy, Roberts said.
Roberts and her colleagues warn that increasing use of marijuana among young women of reproductive age is a major public health concern, especially in light of increased marijuana legalization, for both medicinal and recreational purposes, in the United States.
Another expert agreed.
"If somebody's thinking 'I only smoke a small joint in the morning to help with my morning sickness' -- that's not safe," said Dr. Nathaniel DeNicola, a clinical scholar at the University of Pennsylvania in Philadelphia.
For the study, Roberts and her colleagues evaluated data from more than 5,500 pregnant women in Australia, New Zealand, Ireland and the United Kingdom. About 5.6 percent of those women reported smoking marijuana before or during pregnancy.
The survey only asked women about smoking marijuana, Roberts said. "It is possible they used it in different ways, although since they were recruited between 2005 and 2011 it is unlikely that they were using vaporized marijuana," she said.
The researchers found that mothers who still smoked marijuana 20 weeks into pregnancy were more than five times likelier to suffer a spontaneous preterm birth, after adjusting for age, cigarette smoking, alcohol and their social and economic status.
The harmful effects of smoking pot are one suspected explanation for these results, Roberts said.
But chemical components contained in marijuana, such as THC or cannabidiol, also might play a role, said DeNicola, who is also a clinical associate at Penn Obstetrics & Gynecology Associates.
THC is the chemical in pot that causes intoxication. Cannabidiol, or CBD, does not cause intoxication but does appear to interact with receptors in the human body, experts say.
These chemicals could interact with systems involved in the developing fetus, such as those that provide metabolic support or help the fetal nervous system develop, DeNicola said.
Given that cannabinoid receptors are present in nearly all the body's major systems, "it's not hard to imagine [marijuana] would be related to preterm delivery," he said.
The American College of Obstetrics and Gynecology's advice on marijuana use is similar to that for alcohol and tobacco, DeNicola said -- namely, that expecting mothers should discontinue use during pregnancy.
"Some people may think vaporized or edible products are different, but in terms of pregnancy, we have no reason to believe there's any difference," he said.
The study results were published online recently in the journal Reproductive Toxicology prior to print publication in July.
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