Negative pregnancy outcomes associated with marijuana use, University of Iowa study finds

Minnesota-based study adds 'timely, needed data' on risk, researcher says

Paul Romitti
Paul Romitti

IOWA CITY — As accessibility to marijuana has increased in the United States through recreational and medical legalization in recent years, some studies have shown that Americans are more likely to use the drug than they were in previous years.

However, the long-term impact still is relatively unknown in certain scenarios, including pregnancy and early childhood development, says one University of Iowa researcher.

But a new study from the University of Iowa, along with the University of Minnesota and a Minnesota-based health care system, may shed some light on its effects.

The study, published this month in the Journal of Perinatology, has found that exposure to marijuana during pregnancy may have adverse effects on fetal growth and infant development.

“You combine increased access along with the fact that active substances in cannabis has been shown to enter into fetal circulation, our concern was what impact this might have in fetal development and that it might carry over into childhood development,” said Paul Romitti, senior author on the study and professor of epidemiology at the UI College of Public Health.

The researchers found that women who had cannabis in their system during pregnancy were more likely to give birth to babies who had a low birth weight. Specifically, they were more likely to weigh at or below the 10th percentile, meaning they are smaller than many babies at the same gestational age.

In addition, babies exposed to cannabis during pregnancy may be more likely to have an abnormal developmental screening when they reach 12 months, according to the study.


Researchers found about 9 percent of babies studied who were exposed to cannabis during pregnancy had an abnormal screening. That’s compared with 3.6 percent of babies who had an abnormal screening who were not exposed to cannabis.

The study — funded through a grant from the Centers for Disease Control and Prevention — was based in Minnesota, a state that has legalized medical marijuana but has not legalized recreational use.

Researchers analyzed data of 3,435 women receiving care at 15 obstetric practices within the HealthPartners care system over a 21-month period. Patients not only received prenatal care within the system, but also returned to give birth and for well-child visits when their infant reached nine and 12 months of age.

Each woman received a urine toxicology screening during a prenatal visit, and researchers found 283 women — or about 8 percent of the test subjects — had the psychoactive component of marijuana, called THC, in their system at the time.

Romitti noted that as it was a one-time toxicology screening, the study does not account for continuous use of cannabis by the pregnant mother.

An additional challenge presented to the study was how to account for so-called potential confounders to the data, researchers noted in the publication.

For example, 42 percent of women who were found with cannabis in their system also self-reported they smoked cigarettes during pregnancy.

Research on this subject still is growing but not yet conclusive, Romitti said. The study from the UI does help “add timely and needed data on some of the risk of cannabis use among women in our population.”


Romitti’s goal is to conduct more studies on exposure to cannabis in pregnancy, extending data collection well into childhood, among other measurements.

According to a national survey from 2017, marijuana use among U.S. adults more than doubled between 2001 and 2013.

And with it, so did the use among mothers-to-be. Almost 4 percent of pregnant women reported they used marijuana in the past month in 2014, compared with 2.4 percent in 2002.

The increased use of marijuana among Americans may be caused in part by a shift in public perception, Romitti theorized.

“I don’t know if it’s more about access or more about the misinformation about the safety of cannabis that may be growing faster than the access,” he said.

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