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The risk is also increased by smoking cigarettes, using other legal and illegal drugs and being exposed to secondhand smoke. Stillbirth risk is heightened whether moms are exposed to pot alone or in combination with other substances, the study authors added.
They found that 94 percent of mothers who had stillborn infants used one or more of these substances.
"Even when [findings are] controlled for cigarette smoking, marijuana use is associated with an increased risk of stillbirth," said lead researcher Dr. Michael Varner, associate director of women's health, obstetrics and gynecology at University of Utah School of Medicine.
Stillbirth refers to fetal death after 20 weeks of pregnancy.
Among drugs, signs of marijuana use was most often found in umbilical cord blood from stillborn infants, Varner noted.
"Because marijuana use may be increasing with increased legalization, the relevance of these findings may increase as well," he added.
Indeed, this seems likely as the push to legalize marijuana has gained momentum. Colorado and Washington state voted for legalization of marijuana and states including California, Connecticut, Maine, Nevada and Oregon are legalizing its medical use. In addition, these and other states, including New York and Ohio, are decriminalizing its use.
"Both obstetric care providers and the public should be aware of the associations between both cigarette smoking, including passive exposure, and recreational/illicit drug use, and stillbirth," Varner said.
Although the numbers were smaller for prescription narcotics, there appears to be an association between exposure to these drugs and stillbirth as well, he said.
The report appears in the January issue of Obstetrics & Gynecology.
Study senior author Dr. Uma Reddy, a medical officer at the U.S. National Institute of Child Health and Human Development, said the reason why marijuana may increase the risk for stillbirths isn't clear.
"We don't know, but it's clear there is an increased risk of stillbirth with marijuana. Some of it is overlapping with smoking cigarettes, and we know that cigarette use is also associated with stillbirth," she said.
The more a woman smokes, the higher the risk, Reddy said. For women, Reddy has a simple message: "Don't smoke. If you smoke, stop. You should not use marijuana during pregnancy."
Dr. Jill Rabin, chief of ambulatory care, obstetrics and gynecology at the Long Island Jewish Medical Center in New Hyde Park, N.Y., echoed Reddy's comments.
"We don't want our patients, either before they become pregnant or during pregnancy, to either smoke or use anything that is not medically necessary, like marijuana or even prescription drugs," said Rabin.
For the study, Varner's team analyzed 663 stillbirths that occurred between March 2006 and September 2008. They compared these with about 1,900 live births. For their analysis, they tested umbilical cord blood and blood from mothers for a variety of illegal drugs.
In addition, they asked the mothers about their tobacco and drug use, and looked for signs of tobacco use in mothers' blood samples.
They found that in 94 percent of the stillbirths tested, results were positive for an illegal drug. The most common drug found was marijuana, which was associated with a 2.8-fold increase for stillbirth.
Yet, how strong the association is between all these different drugs and stillbirth isn't easy to pin down, another expert commented.
"In pregnancy it's difficult to determine the exact cause of things, but there is clearly some connection there," said Dr. Ryan Walter, an obstetrician and gynecologist at Scott & White Healthcare in College Station, Texas.
Although all the evidence isn't in, Walter also advises women not to smoke, use drugs or drink when planning to become pregnant or when expecting.
The same is true for secondhand smoke, he said: "It's probably best not to be around it, but if you are married to a partner who smokes or you're in a family of smokers, it's going to be difficult to manage."
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