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Researchers say that smoking both marijuana and tobacco cigarettes while pregnant may be worse for an unborn child than either cigarettes and marijuana alone.

Smoking cigarettes during pregnancy already poses many health risks to an unborn child. Researchers at Baylor College of Medicine say that smoking both marijuana and tobacco cigarettes while pregnant may be worse than either cigarettes and marijuana alone.

The research study, published in the American Journal of Obstetrics and Gynecology, found that women who smoked both marijuana and cigarettes during their pregnancy were twice as likely to experience symptoms such as premature birth, low birth weight and maternal asthma and to deliver an infant with a small head circumference. In addition, the researchers found that while pregnant women who smoke cigarettes generally have less preeclampsia, this was negated when they co-used marijuana.

Researchers collected data from 12,069 women between January 2011 and June 2015 and found that less than 1 percent reported using marijuana during pregnancy, and 48 women reported using both marijuana and cigarettes.

“What we have observed in this initial study is that the combination of marijuana and cigarette smoking results in a higher rate of adverse pregnancy outcomes than either alone,” said Dr. Kjersti Aagaard, assistant professor of obstetrics and gynecology at Baylor and senior author on the study. “This would include both the short- and long-term risks. At this stage, what we know is that marijuana use in pregnancy is not known to be risk-free, and more research is needed in order to understand the short- and long-term impact of use.”

Marijuana is believed to be the most commonly used dependent substance during pregnancy, with reported rates estimating between 2 to 11 percent of pregnant women. With the recent shifts in the legal use of marijuana, researchers want women and their families to make informed choices about the use of medications and substances during pregnancy.

Although there have been previous studies about marijuana use, researchers are only now beginning to learn about the effects of marijuana use in pregnancy. The current research specifically focused on how it is commonly used, which often includes use among women who also smoke cigarettes.

“Just as we once erroneously thought that there was no risk of cigarette smoking because it was legal, we must not assume that legalization or decriminalization of marijuana implies no risk to women or their developing infant,” said Aagaard. “We are fortunate that in our local population of women studied, overall the rate of cigarette smoking and marijuana use was low.” 

This research is currently available on PubMed.

This work was led by Kristin Chabarria and Diana Racusin. Other contributing authors to this study include Kathleen Antony, Maike Kahr, Melissa Suter, and Joan Mastrobattista. All authors are members of the Department of Obstetrics and Gynecology and the Division of Maternal-Fetal Medicine at Baylor College of Medicine and Texas Children’s Hospital. This work was supported in part by Texas Children’s Hospital.