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Consuming Cannabis During Pregnancy: Here’s What the Science Says

Published on March 11, 2018 · Last updated July 28, 2020

In December 2017, a research letter published in JAMA, the Journal of the American Medical Association, reported that marijuana use among pregnant women had increased by several percent between 2009 and 2016. Reuters published a story headlined, “Pot-Smoking on the Rise Among U.S. Pregnant Women.”

The news went national, spreading worry among health professionals and confusion among pregnant women.

'Pot-Smoking on the Rise Among U.S. Pregnant Women' said a recent headline. But that's not what the study actually found.

The scariest quote in the story was provided by Dr. Marcel Bonn-Miller, a researcher at the University of Pennsylvania Perelman School of Medicine, who was not involved in the JAMA study: “The more we study cannabis use during pregnancy,” said Bonn-Miller, “the more we are realizing how harmful it can be.”

But if you read through the actual JAMA study, you’ll find that it didn’t cover pregnant women in the United States, but rather a small set of women in Northern California—many of whom may not have realized they were pregnant when they consumed cannabis, and some of whom may have stopped once they confirmed they were pregnant.

In that same Reuters story, Bonn-Miller acknowledged that “we are just scratching the surface in terms of understanding cannabis use in pregnancy.”

The entire story was alarming and confusing.

The concern is understandable: The major component in marijuana, delta 9-tetrahydrocannabinol, or THC, has been shown to cross the placental barrier and enter the fetal bloodstream. So it’s not unreasonable to wonder how THC and other cannabinoids affect a developing fetus.

The confusion among pregnant women speaks to a pressing and difficult question: How do THC and other cannabinoids affect pregnant women and the long-term health of their children?

The three-word answer to that question is this: We don’t know.

There are no easy or definitive answers when it comes to the relative safety or advisability of cannabis consumption during pregnancy. Doctors, nurses, and other medical professionals are typically reticent when it comes to conversations about cannabis. They either don’t want to discuss it or sternly reject all consumption out of hand. Some immediately turn the conversation into a lecture on criminal behavior.

With little to no information coming from their doctors, it’s not uncommon for pregnant women to turn to personal friends and acquaintances, or the internet, for answers.

Too many doctors won't discuss it rationally, so pregnant women are left to do their own research.
Too many doctors won't discuss it rationally, so pregnant women are left to do their own research.

One Woman’s Experience

That’s what Andrea did. A 26-year-old college student in North Carolina, Andrea gave birth six months ago to a healthy daughter. During the first half of her pregnancy, she took cannabis microdoses from a water pipe to help with early-term nausea and insomnia.

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“Marijuana can definitely be a tool,” she told me, “but using marijuana while you’re pregnant is more than just smoking and doing what you did before you were pregnant.”

She continued:

“You’re taking care of another being, and you need to treat it as such. Like, the cleaner the better. Some marijuana users are constantly smoking joints and blunts, and you know that’s not as clean as it would be in a water pipe, which is what I smoked out of. And, if tobacco’s involved, that’s a whole different ball game… and then there are many more things that you to be paying attention to, like: Have you eaten today? Did you drink water? How are you smoking? Who are you around while you smoke? Does it make you be around a bad crowd? Are you putting yourself in a dangerous situation to go and buy it?”

These are the sorts of questions a health care provider should be able to discuss with a pregnant woman. But Andrea said she had to find most of her information elsewhere.

Divulging cannabis use, especially in prohibitionist states, can carry enormous risk for pregnant women.

“I’m three years into a biology degree,” she said, so she knows how to read a research study. “But there’s no long-term evidence based on studies done on marijuana and the [holistic] effects on babies. There’s a lot of anecdotal evidence, so I was just looking for that [when I was making the decision to consume cannabis.] I dove in online I did some research and asked some people in my life that had smoked with their kids. And, you know they were like, ‘We have had no problems.’”

Online, Andrea said she found a lot of women writing that they and their babies were going to be fine.

Though they offered little advice to her about how to manage her cannabis consumption during her pregnancy, Andrea did tell her doctors about her use. “When I first went to the OBGYN I was assigned to in my county, I did let them know that I was smoking marijuana because I was worried about the social services aspect. And they said really, they never really care or check for that. The doctor said for the most part they’re really checking for harder things that are obviously going to affect the child, the things [that have] a grand amount of evidence [to back it up],” she said.

Why Don’t We Know More?

That’s one woman’s experience, and it should not be taken as universally applicable. The lack of conclusive scientific information regarding the effects of cannabis does not mean that women should smoke marijuana during pregnancy. As with any unknown, it makes sense to proceed with caution.

Andrea’s story, though, led me to ask more questions about cannabis and pregnancy. Why don’t we know more about the effects?

My search for answers sent me to the University of Washington’s Health Sciences Library, where an extensive search through its PubMed database and stacks of medical journals helped me realize that research on cannabis and pregnancy is itself in its infancy. Peer-reviewed studies on the subject only extend back to the late 1980s. By contrast, research on the effects of alcohol on pregnant women goes back as far as the late 1880s.

Playing Scientific Catch-Up

So, we’re starting from a deficit. The first research into cannabis use and fetal health began less than 30 years ago.

Moreover, the nature of that research itself has been doubly limited. For decades, scientists interested in studying the effects of cannabis have had to overcome significant roadblocks set up by the federal government.

What’s more, directly tampering with a fetus or a pregnant mother’s health in the name of scientific research crosses into problematic ethical territory, and is not allowed by the rules of most credible professional associations and institutional review boards (which review the ethical implications of proposed research studies).

Inconsistent Data From Studies

As a result, scientists are left to rely on self-report surveys and biochemical analyses. These are the two least invasive research methodologies, but both methods introduce complications in the study data that can produce inconsistent or downright misleading results.

The authors of a 2002 study in the International Journal of Obstetrics and Gynaecology noted:

“Although the literature on the effects of maternal cannabis use during pregnancy has been steadily growing, it has a number of limitations which include: the use of relatively small samples; the failure to provide estimates of the extent of maternal cannabis use; lack of prospectively collected measures of cannabis use; and failure to control for factors that may potentially confound the association between cannabis use and pregnancy outcomes.”

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Drawbacks in Self-Report Studies

Self-report studies often derive data from questionnaires filled out by expectant mothers, as was the case with Kaiser study referenced in the Reuters report last year. In this methodology, data is entirely dependent on the honor system: Researchers proceed on the hope that expectant mothers will be honest about their own personal cannabis use.

Any mention of cannabis use was, in most cases, ignored or treated as the confession of a crime. The doctor's response changed depending on the pregnant woman’s skin color.

Divulging that information, especially in prohibitionist states like Kansas, Idaho, or Texas, can carry enormous risk for the expectant mother. In many states, cannabis use by a parent has resulted in arrest and custodial loss of his or her children. As a result, many pregnant women underreport, leaving the researchers with flawed data.

“Self-reports are less invasive and permit the evaluation of substance use over longer periods in time, but are influenced by possible reporter social desirability and forgetfulness,” wrote the authors of a 2011 study in European Addiction Research that compared self-reporting by expectant mothers to the results of their urinalysis exams. “Although multiple studies on consistency and validity of multiple assessment methods among adults and adolescents have been reported, little information is available on the agreement between self-reported cannabis use and urinalysis in pregnancy.”

The Risks of Self-Reporting

By mentioning “social desirability,” that 2011 article touched on another confounding part of the study of cannabis and pregnancy: socio-demographics.

The risks of self-reported cannabis use are much higher for mothers of color. In the United States, African-Americans are four times more likely than white people to be arrested for cannabis, despite similar consumption rates. That affects a study subject’s decision to provide, alter, or withhold data. Moreover, for those with lower socioeconomic status, marijuana may be one of the only viable options for treating first-trimester nausea, sleep problems during pregnancy, and other issues.

Troubling Racial Differences

Take, for instance, the alarming data found in a 2017 study by researchers out of University of Maryland and Virginia Commonwealth University, which explored the way medical providers counseled expectant mothers during prenatal visits. Any mention of cannabis use was, in most cases, ignored or treated as the confession of a crime. The medical providers’ response changed depending on the pregnant woman’s skin color.

'Medical providers do not appear to provide adequate counseling' about cannabis use during pregnancy.

“Indeed, providers do not appear to provide adequate counseling,” the researchers found. “In a recent study evaluating providers’ responses when patients self-disclosed cannabis use during a prenatal care visit, 23% of providers did not even acknowledge the disclosure and 48% provided no specific counseling regarding cannabis and its effects on pregnancy. 70% of the time was spent on punitive content such as legal implications and investigations by child protective services. Notably, African American patients were nearly 10 times more likely to receive punitive counseling.”

Conversely, women in higher socioeconomic brackets who report marijuana use may have healthier pregnancies for reasons completely unrelated to their cannabis consumption.

“For example,”wrote the authors of a 2011 Pediatric Research study out of The University of Queensland and Mater Medical Research Institute in Brisbane, Australia,“ it can happen that those who are not typical cannabis users (e.g., those from a higher socioeconomic background) may be more likely to underreport drug use. If this is the case, underreported cannabis use among women who maintain better birth outcomes may lead to overestimation of the association between cannabis use and adverse birth outcomes.”

About That JAMA Research

With all that in mind, let’s circle back to the JAMA research letter reported by Reuters in Dec. 2017. There’s a lot to unpack there.

In that study, researchers examined four million pregnant women throughout California, treated in the Kaiser Permanente Northern California (KPNC) network.

The location of the study immediately raises cultural, historical, and socioeconomic questions.

California features an unusually progressive, racially mixed, and socioeconomically diverse population. Also, California was the first state to legalize medical marijuana in 1996, and California’s residents, on average, embrace legalization at a much higher level than the rest of the country. (On the 2016 ballot, 57% of Californians voted to legalize marijuana.) Plus, Northern California—where the study’s lead author, Kelly Young-Wolff, and senior author, Dr. Nancy Goler, are located—was the locus of early pro-legalization activism.

And yet, headlines accompanying stories about the Northern California-based research seemed to imply that the results applied to pregnant women throughout the entire country.

NorCal Does Not Equal America

Though only three states prohibit both recreational and medical marijuana outright, cultural attitudes towards the use of the drug are different in more conservative states. According to the 2016 SAMHSA National Survey on Drug Use and Health, “Based on 2013-2014 data, 7.22 percent of adolescents aged 12 to 17 across the nation used marijuana in the past month. Adolescent marijuana use ranged from 4.98 percent in Alabama to 8.74 percent in California.”

In other words, not accounting for variable demographics—either during the study or after, as news of the study is consumed by the public—can drive unreliable conclusions about the effects of cannabis.

Pregnancy Awareness Also Matters

There’s another dynamic at play here. The Reuters headline, “Pot-Smoking on the Rise Among U.S. Pregnant Women,” clearly implied that more American women were sparking up during the full term of their pregnancy.

Many women may be consuming cannabis before they realize they're pregnant.

In fact, the data implied nothing of the sort.

The Kaiser Permanente patients in the study were asked about and tested for cannabis use only once—during their initial screening for prenatal care. That screening came at approximately 8 weeks’ gestation time (earlier for some, later for others). Most women don’t realize they may be pregnant until they’re 4 to 7 weeks along. In other words: Many of the women in the study may not have known they were pregnant when they consumed cannabis.

A urine screening, meanwhile, can turn up cannabis use from two to three weeks previous.

So, subjects in the Kaiser Permanente study might have consumed cannabis weeks prior to realizing they were pregnant. Some may well have continued to use cannabis after their pregnancy confirmation—but many may have chosen to stop. We don’t know, because the Kaiser study did not test for cannabis use after that first prenatal screening.

So, What’s a More Reliable Study?

Ready for more complications? Large-sample longitudinal studies using biochemical sampling (analyses of blood or urine), like the Kaiser Permanente study, are the most trustworthy research designs. And these aren’t without their own issues. Prenatal cannabis consumption can often correlate with tobacco and alcohol use, making it difficult to isolate the effects of cannabis use alone.

A mother’s cannabis tolerance, and the lingering detectability of cannabis, can affect the data.

Also, a mother’s tolerance, and the lingering detectability of cannabis, can affect the data. Cannabis can linger in blood and urine weeks to months after use, picking up use that may have happened before the mother was even pregnant. Additionally, these sorts of biochemical studies are expensive, which can impair the research even further.

Some of the limitations on these studies “include small or highly selected samples; lack of prospectively collected measures of cannabis use; and lack of control for potential confounders, e.g., socioeconomic status and maternal health, and also use of other substances (cigarettes, alcohol, and other drugs),” wrote the authors of a 2011 study from The University of Queensland and Mater Medical Research Institute in Brisbane, Australia. “Cannabis users differ from nonusers in a range of ways and any observed association with ever using cannabis may be indicative of residual or uncontrolled confounding.”

“Consistent and Inconsistent”

Even the best research literature often leaves medical providers with conclusions like this one, from a 2012 study also conducted in Brisbane, Australia, on fetal birthweight and cannabis:

“The findings of this study are consistent with previous research, which has suggested that smoking cannabis during pregnancy may lead to lower birth weight, increased rate of premature birth, and shorter birth length. However, our data are inconsistent with findings of studies that have not found a significant association between cannabis use in pregnancy and birth outcomes. The existing discrepancy between the findings might be due to the difference in the study design and assessment of cannabis use (e.g., retrospective vs. prospective) or level of adjustment for confounders.” (This study was published in the journal Drug and Alcohol Dependence.)

To paraphrase in plain English: Our study agreed with some previous findings, and disagreed with others. We attribute this to the problems associated with obtaining reliable data on the actual consumption of cannabis among pregnant women, and the difficulty of isolating cannabis use from other factors like tobacco and alcohol use.

Enter the Precautionary Principle

When the science falls short on hard conclusions and specific details, medical professionals often fall back upon the precautionary principle, which is a risk management strategy used in situations where scientific understanding is incomplete.

One common definition of “precautionary principle” strategy is:

When human activities may lead to morally unacceptable harm that is scientifically plausible but uncertain, actions shall be taken to avoid or diminish that harm. Morally unacceptable harm refers to harm to humans or the environment that is threatening to human life or health, or serious and effectively irreversible, or inequitable to present or future generations, or imposed without adequate consideration of the human rights of those affected.

When medical practitioners apply the precautionary principle to cannabis and pregnancy, the most common recommendation is to abstain, or to avoid mention of cannabis in prenatal visits all together. That leaves pregnant women like Andrea back at square one—turning to friends and the internet for information.

What About Lower Birth Weight?

One of the few areas that has seen more extensive study is the effect of cannabis on birth weight. Even there, though, the research is complex and inconclusive.

One study says birth weight is affected by cannabis consumption. Another says it isn't.

One of the most respected studies to date is “The Health Effects of Cannabis and Cannabinoids,” a comprehensive report released by the National Academy of Sciences in 2017. In that report (which Leafly science director Nick Jikomes discusses here), the authors conclude that smoking cannabis during pregnancy is associated with lower birth weights, while the relationship between cannabis use and other outcomes is unclear.

That conclusion was contradicted, however, by a 2016 study in the journal Obstetrics & Gynecology, which concluded that cannabis poses no significant risk to birth weight and preterm delivery. That finding was consistent with a previous 2010 study funded by the Centers for Disease Control.

The authors of the 2016 study wrote:

“We found that maternal marijuana use during pregnancy is not an independent risk factor for low birth weight or preterm delivery after adjusting for factors such as tobacco use. There also does not appear to be an increased risk for other adverse neonatal outcomes such as SGA and placental abruption once we account for other influencing factors.”

One thing to keep in mind: The National Academy’s 2017 report specified that it wasn’t necessarily the cannabinoids in the smoked cannabis that led to lower birth weight. The authors of that report pointed out that non-cannabinoid byproducts of combustion commonly found in smoke (including carbon monoxide) can impair fetal growth.

How About Cannabis and IQ?

Although this newest review may help relax concerns relating to birth weight and preterm delivery, there may still be other complications attributable to maternal cannabis use, especially in regards to neurological and behavioral development.

One study found that prenatal THC exposure had no effect on IQ, but may have had subtle effects on attention and memory.

A 2013 study in Pediatrics, conducted by researchers at the University of Florida and Harvard University, “indicates that prenatal marijuana exposure is associated at 10 years of age with inattention and impulsivity and with subtle learning and memory deficits. Prenatal marijuana exposure does not appear to affect overall IQ, but it has been associated with underachievement in reading and spelling at age 10 years.

As well, a 2017 Toxicology study from University of Sao Paulo found evidence that THC exposure during pregnancy affected brain development in mice on the molecular level. As their research showed, fetal development is an intricate process involving specifically-timed signaling and THC may have some impact on that process, leading to impairments later in life. Several other studies have echoed these concerns.

These studies also acknowledged that “the literature available evaluating academic achievement is limited.” They also noted that “no independent effect of prenatal marijuana exposure on growth has been documented throughout early childhood and adolescence.”

Tough to Isolate the Effects of Cannabinoids

So we’re back to the issue with confounding variables, and an inability to isolate just for marijuana’s affects as the child in question matures. Many kids who are slow to hit a few academic milestones go on to lead productive and happy lives, which begs the question: How does one measure achievement and so-called normalcy? Clearly, human-made IQ tests are as rife with error and subjectivity as self-reported marijuana use.

On top of that, a 1992 study from Carleton University, Ottawa, Ontario, Canada, found that prenatal marijuana exposure had no measurable impact on a baby’s development, and one groundbreaking study done in Jamaica by Dr. Melanie Dreher in the 1980s discovered babies who were exposed to marijuana prenatally actually performed better on tests of their reflexes than babies in the control group.

So now marijuana could actually be beneficial to a pregnant woman and her baby?

And Then, Add Politics to the Mix

Confused? Well, along with the aforementioned methodological complications, there’s a more political reason why that the research around maternal marijuana use is so baffling.

The circumstances around Dr. Dreher’s study showed a conscious suppression of research that didn’t indicate the “right” things for the National Institute on Drug Abuse (NIDA). According to Herb, after Dr. Lehrer found some positive effects of marijuana use prenatally, her research was defunded, and other factors were put in place that would ensure the results the institute was hoping for:

“Dreher’s research was originally funded by the National Institute on Drug Abuse (NIDA), but after the results of the five-year study showed no statistical difference between the two studied groups, NIDA took away her funding…Instead, NIDA suggested that she continue her research under the supervision of approved scholar, Peter Fried, who’s previous work had found that marijuana harms fetal development.”

This situation implies there could be a more sinister reason why mothers and providers are being jerked around with contradictory research: Big Pharma profits from marijuana’s continued marginalization.

Real-Life Decisions

All these hedged, contradictory, and sometimes confusing studies leave pregnant women in a difficult situation: Wanting to do the right thing, but unable to find conclusive information on which to base their health decisions.

In the case of Andrea, the 26-year-old North Carolina student, she decided to taper off her cannabis consumption later in pregnancy because she was nervous about the effects THC could have as more neurodevelopment occurred.

“I slowed down there at the end especially when development gets a little bit more important, I guess you could say. More neurodevelopment and just like bigger growth steps in shorter amount of time,” she said.

No Easy Answers: Weigh the Evidence

Do we need to have this immense fear reaction if a pregnant woman decides to consume cannabis? That seems like the criminalization of weed rearing its ugly head, especially as other expectant mothers continue to consume substances like caffeine, with effects on fetuses that are similarly fuzzy.

Andrea’s daughter was born weighing a healthy 9 pounds, 6 ounces, and has hit all her developmental milestones since birth. It’s impossible to know how healthy this little girl will be as she grows up, but for now, things look positive.

“We’ve had literally no problems,” said Andrea. “My pediatrician even said out of all the babies that she sees, my daughter is probably one of the most aware.  She says she’s ‘one of those little aware geniuses.’”

Despite all the hype in the media, Andrea says she doesn’t regret a thing. “I would totally do it again. It was so helpful during the beginning of my pregnancy…I don’t think marijuana [in moderation] is dangerous at all.”

That’s one thought. Here’s another:

A report released by the University of Washington’s Alcohol & Drug Abuse Institute (ADAI) in 2016 said, “The science is incomplete, but the public health message is clear: To have the healthiest baby possible, avoid using marijuana, alcohol, and tobacco during your pregnancy.”

Further Reading: Books on Medical Marijuana

The Pot Book: A Complete Guide to Cannabis, by Dr. Julie Holland
The Cannabis Health Index: : Combining the Science of Medical Marijuana with Mindfulness Techniques, by Uwe Blesching
Cannabis Pharmacy: The Practical Guide to Medical Marijuana, by Michael Backes
Stoned: A Doctor's Case for Medical Marijuana, by Dr. David Casarett

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Alexa Peters
Alexa Peters
Alexa Peters is a freelance writer who covers music, writing, travel, feminism, and self-help. Her work has appeared in the Washington Post, Paste, the Seattle Times, Seattle Magazine, and Amy Poehler's Smart Girls.
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