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Does Prenatal Cannabis Exposure Cause Birth Defects?


New studies call into question the idea that cannabis use during pregnancy results in cognitive and physical impairment.

The combination of pregnancy and cannabis is a contentious topic in the United States. And, even with limited and flawed research on the topic, pregnant women have too often found themselves in a public policy minefield that can inflict harsh penalties for cannabis use.

But recent research reveals that these fears may be overblown. Two recent studies indicate that cannabis exposure may not be associated with certain birth defects – like low birth weight or cognitive impairment. 

Researchers at UCLA’s Department of Obstetrics and Gynecology completed the first study, looking at data from California hospitals between July 2016 and December 2018. By looking at the results of urine drug tests during the years that followed legalization, they found that rates of women using cannabis during pregnancy rose from six to 11 percent. 

Given this increase in cannabis exposure, researchers might have expected to see an increase in maternal medical conditions or negative birth outcomes. But so far that hasn’t been the case. 

According to the study, during that time period of rising cannabis exposure, “there was no statistical difference in rates of preterm birth, small for gestational age, NICU admission, or Apgar scores when adjusted for other risk factors.”

In fact, “there were no differences in neonatal outcomes between users and non-users.”

And that’s not all. A second study was recently published in the journal Frontiers in Psychology looking specifically at the impact of cannabis exposure on cognitive impairment in children.

The authors of this study, from Columbia University and Australia’s Swinburne University, explicitly highlight public policies that reflect the belief that cannabis is toxic to the developing fetus. 

Neither study suggests that pregnant women should not exercise caution with regards to marijuana. They do both question whether our fears around prenatal cannabis exposure are backed by good evidence.

In an interview with Forbes, Dr. Ciarra A. Torres said that she was disturbed to learn “that individual studies have often been cited by US and state legal and medical authorities when implementing policies that serve to punish mothers — as by incarcerating them, or removing children from the home.”

“I wanted to see if the huge concerns we’ve had [as a society] around prenatal marijuana exposure were warranted,” she explained.

So her team reviewed data from 40 studies that date back as far as 30 years. All of these studies looked at how prenatal cannabis use affected children’s cognitive functions compared to children who had had no exposure to the substance.

Researchers then took these numbers and compared them to a normative range. This is an average of cognitive scores that have been controlled for factors like age and educational level. This is an important step that helps provide context for just how significant differences between scores really are.

What they found was that the correlation between cannabis exposure and cognitive impairment seems to have been inflated. In fact, Cognitive scores for children who had been exposed to cannabis were below the normal range in only 0.3 percent of the total sample.

“Thus,” the researchers wrote, “despite analyzing studies spanning approximately three decades, we conclude the evidence does not support an association between prenatal cannabis exposure and clinically relevant cognitive deficits.” 

Researchers did point out some issues with the numbers they were working with. For one thing, most of the studies they reviewed didn’t actually confirm how much cannabis mothers were using or confirm cannabis exposure by drug testing.

Other variables further complicate the findings. For example, the study authors note that cannabis use during pregnancy is often associated with a long list of risk factors for children that go beyond direct exposure. This makes it difficult to parse out what is causing damage to a developing fetus. 

“A greater understanding is necessary of the fact that many children with prenatal cannabis exposure are also exposed to factors often seen in people with low socioeconomic status, such as poor nutrition, parents with lower levels of education and parents who may also use other substances, including nicotine and alcohol, among a host of other confounding variables.”

We should note that the study does not rule out that cannabis may negatively impact the cognitive development of a fetus, but only that the research to date has failed to show evidence of it. And they point out the possibility that future studies “may yield a more concerning pattern of effects. ”

Essentially, no researchers are suggesting that pregnant women should throw caution to the wind when it comes to cannabis use. However, both studies point to the need for further, robust research on cannabis and pregnancy. And they highlight a need for public policy that follows, rather than proceeds, those findings. 

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