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Home » Part 5. Marijuana and pregnancy – assessing the science

Part 5. Marijuana and pregnancy – assessing the science

Last updated on October 1st, 2020 at 12:43 pm

This is Part 5  of a series of six articles discussing various medical uses for cannabis or marijuana. In this article, I review and summarize some of the evidence that marijuana supporters have used to claim that smoking weed during pregnancy is safe. And I look at data from high quality reviews that shows it isn’t. So let’s assess the science regarding marijuana and pregnancy.

As I have written previously, consumption of and growing marijuana should be completely decriminalized. And the laws need to be rewritten, not in the haphazard way it is now, but with protection and respect of rights of people to consume or grow (for personal use) cannabis. The criminal prosecution of marijuana use and distribution is a ridiculous waste of public resources.

There are numerous health claims made about marijuana, including its use for cancer, neurological disorders, and other medical uses. There’s even a whole area of belief that claims smoking cannabis is perfectly safe during pregnancy. Let’s look at this.

Claims about marijuana and pregnancy

Recently, I was pointed to some “pro-marijuana” websites that essentially claim that smoking cannabis was safe during pregnancy. I was flabbergasted.

The pro-marijuana lobby group, NORML (the National Organization for the Reform of Marijuana Laws), published an article in 2010, that claimed:

The Occasional Toke: Safe for Moms or Not?–Unfortunately, there is not a definitive answer to this question available at this time. Nevertheless, with a reported three percent of US women of reproductive age reporting that they occasionally use marijuana, it may be arguable that potential pre-natal and post-natal dangers posed by maternal pot use – particularly moderate use – are rather minimal, especially when compared to the in utero exposure of alcohol and tobacco.

The NORML article then cites 15 published scientific articles as support for their conclusions. But do these articles really imply that marijuana and pregnancy is a safe combination?

Let’s take a look at their supporting evidence:

  • Some of the research is dated. It’s not that old research is necessarily bad, but if one is going to make a serious claim that cannabis use is safe, then bring the most modern and best research possible.
  • Three of the articles are published in journals with an impact factor of 0.00, which means that over the past five years, no one cited any article in those journals. Well-done, well-respected research is published in journals with high impact factors and are cited frequently, up to 100 (and even higher) times a year.
  • One of the recent articles, published in the the British Journal of Psychiatry, showed an odds ratio of approximately unity, meaning that there was no increased risk of psychotic behavior in children of mothers who smoked pot. But the article states that there really were insufficient numbers of women who smoked cannabis during pregnancy to determine if there was a possible risk difference depending on which trimester (which is supported by other studies). In other words, NORML used a source which really doesn’t support their case of safety.
  • Another of the articles, published in one of those 0.00 impact factor journals, makes claims about the usefulness of smoking marijuana to reduce morning sickness. However, it was a terrible study (what do you expect for a 0.00 impact factor journal) which accumulated self-reported results without any serious controls, randomization or blinding. Moreover, the study did not examine the effect of the smoking on any parameter of fetal development. I actually found no evidence that the researchers had any kind of ethics review board approval of this research, and the first author is a proponent of herbal medicine, a mostly pseudoscience.
  • The list of research citations is the perfect example of cherry-picking. They find the studies that support their point of view, when real science looks at all the research, ranks them from best to worst, and see where the evidence goes. But we’ll get to that.


The infamous Jamaica study

This study, published in 1991 in a respected journal, Pediatrics, is quoted frequently in support of smoking cannabis during pregnancy. NORML uses it. At least 100 memes I found on the internet mentioned this article.

Essentially, the study compared the outcomes of 24 children born to mothers who “used” marijuana to 20 children whose mother did not. The researchers concluded that “the neonates of heavy-marijuana-using mothers had better scores on autonomic stability, quality of alertness, irritability, and self-regulation and were judged to be more rewarding for caregivers.” Sounds pretty good.

But let’s examine the evidence presented for quality, because that’s what we should do.

  • This study has only been cited by 5 other articles in the 24 years since being published. If this research was so compelling, so critical, it would be repeated in other populations and published in other articles.
  • A population of only 44 children, roughly divided between the user and non-user groups, is too small to make a clinically meaningful conclusion.
  • There were no controls for confounders (like type of family environment, nutrition, other health issues). Of course, controlling confounders is nearly impossible with just an n=44 population size.
  • Another similarly designed study in Jamaica contradicts the results of Pediatrics study. This is why we don’t cherry pick data–we pick the best evidence published in the best journals to form a conclusion.
  • Ironically, one of the 5 articles in 23 years that did cite the Jamaica study, did its own small study comparing non-user and user mothers and neonatal outcome. The authors concluded that “prenatal cannabis exposure was associated with fetal growth reduction. Meconium testing primarily identifies prenatal cannabis exposure occurring in the third trimester of gestation.” Now I think this studied is flawed in the same way as the Jamaica study, but once again those who advocate that smoking marijuana are cherry picking bad articles, while ignoring others that are less supportive of their point of view.

This so-called “Jamaica study” fails to meet the standards of the highest quality of research, and the results have not been repeated. To use it as the basis of a broad claim that marijuana smoking is safe for fetal development does a disservice to pregnant women. This study is scary bad and is not at all useful in supporting a conclusion about the safety of marijuana use and neonatal outcome.


Breast feeding and marijuana

Since apparently some people think that smoking marijuana post-partum is safe, I thought I’d spend a few moments reviewing the best literature (quality of research matters, so if I’m to be accused of “cherry picking,” it’s picking the best quality).

Here’s what some of that top research conclude:

  1. THC can accumulate in human breast milk to high concentrations.
  2. Infants exposed to marijuana through their mother’s milk will excrete THC in their urine during 2 to 3 weeks.
  3. A large review of the scientific literature regarding breastfeeding and neonatal development showed that breastfeeding while consuming marijuana is “dangerous,” while providing plausible connections between cannabis consumption and several neurodevelopmental issues.
  4. One of the best studies, a a meta-review published in the Journal of Perinatology (a newish Nature journal), Cannabis, the pregnant woman and her child: weeding out the myths (I love a good title), concluded that “women who are using cannabis while pregnant and breast feeding should be advised of what is known about the potential adverse effects on fetal growth and development and encouraged to either stop using or decrease their use. Long-term follow-up of exposed children is crucial as neurocognitive and behavioural problems may benefit from early intervention aimed to reduce future problems such as delinquency, depression and substance use.” In other words, there appears to be high quality scientific evidence that there is a link to certain neuro-developmental issues in children who are exposed to cannabis while breastfeeding.

A lot of people advocate for breast feeding because it’s healthier than formula. Without contributing to that particularly divisive discussion, the thought that cannabis (or any other drug such as cigarettes or alcohol) can get into breast milk and harm the child is particularly scary. Not only do we lack any recent high quality evidence that it’s safe, we actually have real high quality evidence that it’s dangerous.


More high quality evidence

It’s time to move away from critiquing research cherry-picked by marijuana advocates to “support” their beliefs, and move toward research that provides evidence that marijuana and pregnancy is a bad combination.

Let’s examine the highest quality scientific studies–systematic reviews–to investigate what might be the scientific consensus on marijuana and pregnancy. I simply searched PubMed for marijuana (or cannabis), pregnancy (or maternal) and review. Then I read the studies in the best journals with the high quality of data.

  1. A systematic meta-review grading the evidence for non-genetic risk factors and putative antecedents of schizophrenia. Authors concluded that “the risk factors (for schizophrenia) with the highest quality evidence, reporting medium effect sizes, were advanced paternal age, obstetric complications, and cannabis use.” This journal has a relatively high impact factor of 5.056. Furthermore, the authors are well published in schizophrenia research and can be considered authorities in the field. The meta review itself is based 24 published studies. This systematic review is a particularly troubling one about the safety of marijuana consumption while pregnant.
  2. Molecular mechanisms of maternal cannabis and cigarette use on human neurodevelopment. This review article described a plausible biochemical relationship between maternal cannabis consumption and behavior issues of children. The authors concluded “the studies reviewed here emphasize the sensitive nature of the prenatal developmental period, during which cannabis and cigarette exposure can set into motion epigenetic alterations that contribute to long-term disturbances in mesocorticolimbic gene regulation, thereby laying a foundation for increased vulnerability to addiction and potentially other psychiatric disorders.” The researchers lay out a solid biochemical connection between cannabis, and its constituent cannabinoids, and neural receptors in the developing fetal brain. This establishes one of the most important factors in biomedical consensus, biological plausibility.
  3. Chronic toxicology of cannabis. This study analyzed 5198 papers, an impressive review. The author concluded that, “Chronic cannabis use is associated with psychiatric, respiratory, cardiovascular, and bone effects. It also has oncogenic, teratogenic, and mutagenic effects all of which depend upon dose and duration of use.” There’s a lot here that has nothing to do with maternal outcome, but “teratogenic” effects is worrisome, since that implies a dangerous effect on fetal development.
  4. Maternal smoking, drinking or cannabis use during pregnancy and neurobehavioral and cognitive functioning in human offspring. This meta review published in a very high impact factor journal (11.075). Anyways, the authors concluded “that prenatal exposure to either maternal smoking, alcohol or cannabis use is related to some common neurobehavioral and cognitive outcomes, including symptoms of ADHD (inattention, impulsivity), increased externalizing behavior, decreased general cognitive functioning, and deficits in learning and memory tasks.” The authors also stated that “exposure to substances such as alcohol, nicotine and cannabis may produce abnormalities in brain development. The behavioral impact of any such abnormalities that might occur depends on other pre- and post-natal factors, which may include genetic vulnerability.”

Marijuana and pregnancy (or while breastfeeding) is a bad combination–it is probably not very safe to the fetus or neonate. That’s very clear.

See the next article in this series, Part 6, Medical uses for cannabis – the TL;DR version.

Editor’s note: This article is a substantial update of an article (and a few other articles) that was originally published in April 2014. It has been completely revised and updated to include more comprehensive information, to improve readability and to add current research. I’ve also sub-divided into six separate articles to improve readability, and to allow a reader to choose the marijuana and medicine article that is of highest interest.

Another editor’s note: I have a policy of open commenting and dissent to my articles. I only delete spam, racism, and just random nonsense. Please comment here if you wish, because I know everyone has an opinion on marijuana. But ad hominem hatred, strawman arguments and other logical fallacies will be mocked–bring high quality, peer-reviewed evidence. I might even rewrite or add another section if it’s high quality and peer-reviewed.

Key citations:


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