As more states consider marijuana legalization, new research published in the Journal of Perinatology describes how cannabis exposures during pregnancy could affect infants’ growth and development.

Researchers at the University of Iowa, HealthPartners Institute, and the University of Minnesota analyzed data on 3,435 women receiving prenatal care in the HealthPartners care system over a 21-month period. Urine testing, a routine part of pregnancy care in the care system, showed 283 women had cannabis in their system while pregnant.

According to the study, babies born to women who had cannabis in their system during pregnancy were more likely to have their birth weight for age at or below the 10th percentile, also known as small for gestational age. Additionally, 9.1 percent of the babies exposed to cannabis during pregnancy had an abnormal developmental screening at 12 months of age, compared to 3.6 percent of babies who were not exposed to cannabis during pregnancy.

“This research adds to the growing body of literature on the effects of prenatal use of marijuana on fetal and child development,” says Dr. Paul Romitti, professor of epidemiology at the University of Iowa College of Public Health and senior author of the study. “With marijuana use being legalized in more states, the potential for use among women who are pregnant or not aware that they are pregnant has increased.”

The study was funded by a grant awarded from the Centers for Disease Control and Prevention to Dr. Romitti at the University of Iowa.

Although nationwide data show about 5 percent of women self-report using marijuana during pregnancy, over 8 percent of women the researchers studied had a positive urine screen for cannabis during a prenatal visit. According to the researchers, this could indicate that self-reported figures may not capture the full scope fetal cannabis exposures. With the increasing access to cannabis across the nation, some reports suggest that women are using the drug to self-medicate pregnancy-related nausea.

The researchers also note that it’s possible that women might use cannabis when they don’t realize they’re pregnant, they don’t realize how long cannabis can stay in their system, or they might use and don’t report this on surveys due to fear of stigma. They add that the study data points to a need for more comprehensive research of infant growth and development following in utero cannabis exposures and recommend that women who are or may be pregnant, shouldn’t use cannabis products.