Marijuana and Pregnancy: What Does the Science Say?

The best available evidence suggests fears about fetal risk, while not totally unwarranted, are often overblown.


It's 2021 and more U.S. women are using—or at least admitting to using—marijuana while pregnant. In 2016, around 7 percent of pregnant women in a national survey said they used marijuana in the past month, up from 3.4 percent in 2002 and 3.9 percent in aggregated data from 2007-12. Should public health authorities be worried?

Probably not. Shaky science makes measuring the effect of a mom's marijuana use on fetal development difficult, but the best available evidence suggests fears about fetal risk, while not unwarranted, are often overblown.

Studies on marijuana use during pregnancy are inconsistent and inconclusive. But cannabis is not known to be teratogenic—that is, to cause birth defects—in humans. The bulk of scientific evidence suggests that risks posed to developing fetuses are relatively minor and babies exposed to marijuana in utero still fall within normal ranges of outcomes.

A 2020 review looked at longitudinal studies on "the impact of prenatal cannabis exposure on multiple domains of cognitive functioning in individuals aged 0 to 22 years" and found that "evidence does not suggest that prenatal cannabis exposure alone is associated with clinically significant cognitive functioning impairments." Researchers did note some differences—"those exposed performed differently on a minority of cognitive outcomes (worse on < 3.5 percent and better in < 1 percent)" — although "cognitive performance scores of cannabis-exposed groups overwhelmingly fell within the normal range."

A 2016 review of studies on potential ties between in utero marijuana exposure and adverse birth outcomes—things like low birth weight and preterm delivery to miscarriage and stillbirth—found "maternal marijuana use during pregnancy is not an independent risk factor for adverse neonatal outcomes after adjusting for confounding factors." Instead, any increases in adverse outcomes appeared "attributable to concomitant tobacco use and other confounding factors."

Sussing out the effects of marijuana from the effects of other risky behavior presents a problem. "Illegal behaviors tend to cluster so women who use marijuana are more likely to use other drugs," Emily Oster, author of Expecting Better and Brown University economist, explained in her Substack Parent Data. "Reporting on use is hugely biased and we are likely to identify only a (very non-random) subset of women who use."

As marijuana becomes increasingly accepted, studies on its effects are likely to improve. "The best new evidence on this comes from a 2019 study out of Canada," Oster writes. Matching women who used cannabis with demographically similar women who didn't, researchers did "find evidence of worse birth outcomes among the cannabis users," including "an increased risk of prematurity and NICU transfer. The increases are moderate but statistically significant: preterm birth occurred in 10% of cannabis users and 7% of non-users." An August 2020 study from the same authors found marijuana use correlated with slightly higher incidences of intellectual disability and learning disorders, as well as higher chances of having autism spectrum disorder. "The percent increase is large—about 50%—and significant," Oster points out, though the researchers do note that the overall incidence rate is still small.

Though the researchers tried to demographically match participants between groups, it can still be hard to totally compensate for the ways marijuana users may differ from non-users. For instance, marijuana users may be more likely to have partners who use, too, and dads who smoke pot prior to conception may have sperm changes that correlate with higher autism risk.

Ultimately, correlational studies like these raise important possibilities but have major limitations. Some are done not during pregnancy but by later asking women to recall behaviors during their pregnancy, which ups the likelihood of unreliable information. They also often fail to differentiate between consumption methods, timing, and dosage, so we can't know if any and all marijuana use triggers a specific outcome or if it's only produced by certain methods of consumption, THC content, frequency, or at certain stages of pregnancy.

The biggest problem is that it's hard to isolate specific factors like marijuana consumption. The population of women who not only use marijuana during pregnancy but are also willing to admit to researchers that they do may differ from those who don't.

An Australian study published in 2020 found differences in birth size and length, head size, and gestational age at birth in the offspring of women who had still used marijuana at or after 15 weeks. But "the mean age and socio‐economic status of women who continued to use cannabis were lower, and their mean anxiety and depressive symptom scores higher than for other participants; the proportions who consumed alcohol, used other illicit drugs, or were smoking at 15 weeks of pregnancy were also higher." (Notably, "neonatal outcomes for babies of women who quit before or during early pregnancy were not significantly different from those for infants of women who had never used cannabis.")

A review of evidence published in February 2020 "points to the possibility of lower birth weight, diminished IQ and more behavior problems among children whose mothers used cannabis during pregnancy, but notes it is very difficult to separate the marijuana use from other demographics or other variables," writes Oster.

Another 2020 study purported to show children exposed to marijuana in the womb had a range of "attention, thought, and social problems" later in life by looking at data on 9- to 11-year-olds in a large longitudinal study. But since women who admit to using marijuana during pregnancy are different in myriad ways from those who don't, there could be a range of differences in the heritable traits or early childhood experiences of their offspring, which could be responsible for subsequent cognitive differences.

When public health officials in the U.S. say that "no amount of marijuana use during pregnancy…is known to be safe," this should be taken with a heaping grain of salt. Phrases like "known to be safe" or "proven to be safe" are frequently applied to the consumption of all kinds of substances—alcohol, artificial sweeteners, certain foods, prescription and over-the-counter drugs, even nutritional supplements—during pregnancy. This is because studies directly testing the safety of these substances for pregnant women can't ethically be done. Hence, a huge range of legal and illegal substances aren't "known to be safe" for developing fetuses, but that doesn't mean they're known to be harmful.

We know some substances to be unsafe during pregnancy—drinking lots of alcohol, smoking tobacco, consuming too little folic acid. Marijuana isn't one of them.

With the limited evidence available, it may make sense for most pregnant women to avoid marijuana to minimize possible risks to their offspring. But the best choice for one woman and her baby won't be the best choice universally. For women who have extreme morning sickness that makes getting adequate nutrients through food and vitamins difficult, and for whom marijuana mitigates nausea, using cannabis might make sense. Likewise, women with certain mental health conditions helped by marijuana may deem it safer than their usual prescription drugs.

Ultimately, pregnant women should make these decisions for themselves, in consultation with their doctors. Unfortunately, the state doesn't always see it this way.

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  1. It is not enough that cannabis be legal,we must also show it is not harmful in any way.

    I am not sure I understand this need.

    1. Which one of the studies did you find objectionable? Or are you just a prick?

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    2. Apparently we have to show that it is not harmful in any way to convince even Dear Blue Leader that marijuana should be legal.

    3. The attitude currently within medical science is that marijuana + pregnancy = BAD, and when routine toxicology screens done on women in OB reveal the presence of THC they are automatically referred to child protective services, and then are in the system and mandated to get “treatment” for their “drug problem.” If I were to walk down the hall right now to OB and hand this article with all the references to studies to the physicians there they would proclaim “Oh no, current guidelines for practice mandate that fetuses exposed to THC in utero will have low birth weigh, cognitive delays, and respiratory problems.” And that would be that.

      I think what is going on is that there is a perception of cannabis as an outlier, as an illicit substance [only recently legalized in certain locations], as socially and politically unsavory, and we are trying to enforce, and still criminalize as much as possible, a sense of morality about it. And change in that regard takes generations, and the associated law and often the “science” will lag considerably.

    4. I don’t know, but it’s childishly stupid and I wish it would stop (just as I wish “medical marijuana” would stop as well – people should be able to do anything to their bodies as far as I’m concerned, ingest whatever, including marijuana, but stop trying to make this single drug into the super medical solution for everything – just legalize it and move on).

  2. A fetus is just a clump of cells with no rights. Therefore pregnant people should be able to use drugs without any shaming from the extremists who literally want to turn this country into The Handmaid’s Tale.


    1. Obviously.

    2. Depends and what you want to do with that clump of cells; want to get rid of it, it’s nothing, but if you want to birth it, it’s your child. And a government’s responsibility. Capiche?

      1. No. – When it comes to a person’s body, no person or government has the right to claim dominion over it, for any reason. That is the final core of freedom.

        A fetus is not a person. It’s an organism and part of the woman’s body, belonging wholly to that woman. It only becomes a person at birth.

        1. Organism reveal parties.

  3. Shaky science makes measuring the effect of a mom’s marijuana use on fetal development difficult

    “Shaky”, eh? Well, if everyone were following *you*, you’d be shaky, too!

  4. I better call my mom and apologize for saying her doobie use while she was pregnant made me dumb. I’m just dumb and have no one to blame but myself?

    1. Or maybe it’s genetic. After all, your mother used drugs while she was pregnant – and if she did that, she probably also drank. Excessive alcohol in utero _is_ proven to cause neurological damage as well as physical deformity.

  5. What Does the Science Say?

    The science doesn’t speak. It sings. Mommas Don’t Let Your Babies Grow up to Be Deadheads. If that’s not crystal clear medical advice re pot, then your kid is going to die of dysentery on some ashram.

    1. dude I still listen to a portion of a Dead show pretty much every … wait what was I talking about?

    2. No thanks to the childish demonizing. – SAMHSA research has determined more than 100 million Americans have consumed marijuana. About HALF the of-age population.

      There are now more than 60 million current U.S. consumers. The vast majority consume moderately – on the weekends or less, and are successful, hard-working, respected members of their communities. They are from all walks of life and loved by the families they support.

      Since we now know marijuana is less “addictive” than coffee, is about HALF as intoxicating as alcohol (for a much shorter duration of time), is not a significant cause of auto accidents, does not fuel violence like alcohol, and is far less harmful than alcohol, this is no surprise.

      Every person who chooses near harmless marijuana over addictive, very harmful alcohol, improves their health significantly – as well as the lives of their family and community.

  6. It seems like that age old problem of allowing women and their doctors to work out issues. It just always seems better to have old white men making those decisions for the women.

    1. “old white men”

      Trope of the day. And I know plenty of not so old women of various races who are more than happy to make such decisions for others.

      Try something more convincing.

      1. So what state legislature is made up of mostly women?

        1. According to NCSL – Nevada with 52%. Arizona and Colorado are close with 47%. The rest range from 15% to 35%.

          The Colorado one doesn’t surprise me because that’s where I live – but I’m quite surprised at most of the rest. Confirms to me WHY legislatures often froze their representation when suffrage was expanded. Force the new outsider groups into a zero-sum competition for representation. Divide et impera – 100 years later.

          This is exactly what sortition would solve. The law of large numbers would quickly tend to make a legislature approach the demographics of the population – by every possible measure from gender to age to occupation. All without even trying and with no gerrymandering possible.

      2. “Old woman” is an older version of “Karen” – an undesirable character trait that may be found in all genders and ages.

        1. And it does describe the majority of legislators, because that’s why they entered politics.

  7. So the article concludes there is some risk, but not much. How does an expectant mother quantify this? Is ENB actually this dense?

    1. You might reread the article. General rule is the same for all drugs in pregnancy, avoid them. As ENB points out some women may need marijuana for nausea relief or mental health issues and will have to balance the need against the risks. Same as any other medication in pregnancy.

      1. What is missing here, and in most attempts to address marijuana and pregnancy, is where any potential risk might rank on a scale of various other harmful behaviors.

        Poor diet, lack of exercise, and alcohol consumption are not just risks, they are clearly behaviors that damage the fetus.

        Hence, the possibility of “risk” from consuming marijuana can only be placed in the area of recommended guidelines, not regulations.

        There’s little doubt that much of the hysteria the media is trying to gin up over the possibility of risk to fetuses is sponsored by interests that want to maintain the fraudulently enacted marijuana prohibition.

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