A study out this week is the latest to suggest that using cannabis while pregnant isn’t necessarily harmless. Scientists have found a link between a mother’s occasional cannabis use during pregnancy and poorer outcomes for newborns, such as lower birth weight. These effects were more significant in mothers who used cannabis late into their pregnancy but could still be seen in those who used cannabis early on.
The work was conducted by researchers from Central Michigan University’s College of Medicine. A growing number of studies have shown a link between cannabis use during pregnancy and poorer outcomes for newborns. But in this research, the scientists were also able to look at the timing and duration of cannabis exposure.
The team tracked the health of expectant mothers and their newborns who received care at a local hospital, focusing on those who said they had recently used cannabis or tested positive for it at some point of the pregnancy. They were then compared to mothers who never tested positive for cannabis during their pregnancy. All told, about 270 mother-child pairs were included in the study.
As with other studies, the team found that cannabis use during pregnancy in general was linked to smaller size in newborns—an effect that was apparent even when mothers stopped using cannabis by the end of the first trimester. But they also found that mothers who continued to use cannabis while pregnant into their second or third trimesters had smaller children on average, along with other added negative outcomes. The team’s findings were published Tuesday in the journal Frontiers in Pediatrics.
“We show that even when marijuana use occurred only in the first trimester of pregnancy, birth weight was significantly reduced, by more than 150 grams on average,” study author Beth Bailey, professor and director of population health research at Central Michigan University, told Frontier Science News. “If that use continued into the second trimester, newborn head circumference was significantly decreased as well.”
The authors note that newborn size plays an important role in a child’s development. Low birth weight in particular has been linked to a higher risk of many other conditions throughout life, including cardiovascular disease, diabetes, and even a greater vulnerability to diseases like covid-19.
Like any study, this research has its limitations. The findings are based on a relatively small sample size of expectant mothers at a single site, for one. And while the researchers were able to objectively track the timing of people’s cannabis use, they didn’t have data on how often people were actually taking it regularly. Scientists are also still trying to figure out the exact reasons why cannabis might be harmful during pregnancy and whether these effects could differ depending on how it’s taken.
All that said, it seems likely that cannabis does pose a real risk to a developing fetus. And given that recreational cannabis use is becoming more legalized in the U.S. and elsewhere, it’s a risk that expectant parents should be warned about. For years now, organizations like the American College of Obstetricians and Gynecologists have recommended that doctors advise expectant parents not to use cannabis during pregnancy. And the authors say their research only reinforces the soundness of that advice.
“It is important to encourage patients to stop using marijuana as soon as they become pregnant to reduce fetal growth risks and potential long term adverse health and developmental outcomes in offspring,” they wrote. “These findings may also be used to influence commercial practices at cannabis dispensaries where marijuana use during pregnancy has not always been discouraged.”