Illustration: Jim Cooke/GMG

Marijuana has a healthier image than many other drugs (or, depending on who you ask, alcohol), and it can ease symptoms like nausea that tend to crop up in pregnancy. But we are accumulating evidence that marijuana is probably not a great idea to use if you’re pregnant. In particular, the THC component can affect a baby’s brain in ways that you might not notice until they are older.


If you’re looking for a simple yes/no answer on whether it’s okay to use cannabis in pregnancy, I’ll point you to this categorical no from the American Congress of Obstetricians and Gynecologists. “Although there are limitations to the data on marijuana use during pregnancy...worrisome trends do emerge,” they write. Children who were exposed to marijuana in utero are more likely to have cognitive and behavioral problems later in life. The experts I spoke with absolutely did not recommend smoking weed while you’re pregnant. If you want a yes or no, there’s your no.

But you’re probably reading this because you want to know why it’s a no, and how good the evidence is, and whether there’s any wiggle room for your situation. So here are the details.


Marijuana Affects Brain Development

The best known marijuana chemical is tetrahydrocannabinol (THC). It’s the main psychoactive component, the part that gets you high. THC circulates in your blood, and can cross the placenta, so the fetus is exposed to it too. THC is also fat-soluble, and can end up being incorporated into your body fat. That means that you can still have small amounts of THC circulating in your body even if you haven’t consumed any marijuana lately.

THC can also enter breast milk, and its metabolites end up in baby poop, so if you use marijuana and also breastfeed, your child is exposed to the chemical that way too.

Some of what we know about the biology of marijuana comes from studies where scientists give pure THC to animals like rats. Diana Dow-Edwards, who does animal research on how drugs affect the developing brain, says it’s well established that THC interferes with the way brain cells connect to each other. “This of course is the whole essence of the brain,” she says. “One neuron connects to the next neuron, which connects to the next.” And the more marijuana the brain is exposed to, the greater the effect on those connections.


It’s well established that THC interferes with the way brain cells connect to each other.

Human brain cells develop in a similar way to those in rodents, but we need real-world studies to figure out what this means for us. It’s not ethical to do a randomized controlled trial, because that would involve assigning some women to use marijuana in pregnancy. So instead, studies look at the children of people who decided on their own to use marijuana in pregnancy, and try to figure out whether those children have more problems than children whose mothers did not use marijuana.


Only two of those studies, Dow-Edwards points out, followed children for decades after birth and controlled for factors that might otherwise bias the results. They are the Ottawa Prenatal Prospective Study, begun in 1978, and the Maternal Health Practices and Child Development Study, begun in 1982.

Results from both studies showed that the pregnancies and births went more or less as usual, but as the children got older, they were more likely to show problems with memory, attention, and hyperactivity.


But What About Other Studies?

The evidence from those long-term studies, plus what we know in animals, definitely points toward cannabis having detrimental effects on kids’ brains. But there have been enough studies done on the effects of marijuana in pregnancy that it’s easy to cherry-pick a few with the opposite conclusion.


Publications that are friendly to marijuana tend to ignore the studies above, and instead prefer others like this one done in Jamaica. I asked Dow-Edwards how these studies fit in to the larger body of research, and she pointed out that Jamaican mothers, and the way they use marijuana, are different than what you’d find in a US or Canadian population. In Jamaica, ganja is often consumed as a tea, and this tea contains mainly cannabidiol (CBD) and very little THC. The Jamaican studies were also small, and didn’t follow the babies as they grew up.

Another short-term study with superficially good results is this systematic review that looked at whether marijuana contributed to babies being born early or small. It concluded that “marijuana use during pregnancy is not an independent risk factor for adverse neonatal outcomes[.]” Herb summarized this as: “New study says smoking cannabis while pregnant is OK, as long as it’s in moderation,” which is not at all what the study said.


In fact, the study’s authors concluded that, while marijuana is not as bad for newborns as other drugs, “these data do not imply that marijuana use during pregnancy should be encouraged or condoned.” Pot may not hurt newborns in an obvious way, but it’s the long-term consequences that seem to be serious.

Colorado’s Summary of the Evidence

If only somebody had sorted through all the studies, both short and long term, that offered information about the effects of prenatal marijuana on kids’ health. If only that somebody was from a relatively unbiased source like, oh, let’s say, the public health department of a state that allows both medical and recreational marijuana. That would be nice, right?


You can send your thanks to Colorado’s Retail Marijuana Public Health Advisory Committee, which publishes an annual report on the science behind health and marijuana. The chapter on pregnancy and breastfeeding breaks down what we know according to how strong the evidence is. There are no conclusions that have what they call substantial scientific support, but several are in the moderate evidence category, which they define to mean that “scientific findings support an association between marijuana use and the outcome, but these findings have some limitations.” In other words, the evidence is pretty good but not ironclad. These include:

Attention problems

Decreased IQ scores in young children

Decreased cognitive function

Decreased growth

Then there are some findings with limited evidence, which means that studies support a link but the studies have “significant limitations.” Some of these may turn out to be flukes, but then again they may be real:

Stillbirth

Certain heart defects

Decreased academic ability

Increased depression symptoms

Delinquent behavior

This evidence category also holds some good news: there is limited evidence that prenatal marijuana use is not linked to SIDS.


In the next category, there is insufficient evidence to link marijuana use in pregnancy to psychosis symptoms in adolescence or an increased likelihood that the child will grow up to be more likely to use marijuana themselves. There is also insufficient evidence to link marijuana use during breastfeeding with SIDS.

Finally, there’s another category for mixed evidence, which means that some studies say yes and others say no. Most birth outcomes (preterm birth, low birth weight) fall into this category. You can check out the full report to read about which studies the advisory committee used and how they analyzed them.


Not All Weed Is the Same

Why are these studies all coming to different conclusions? Besides the difficulty of doing a study at all (and getting women to admit that they have used marijuana in pregnancy), there are also a lot of what researchers call confounders. If you use marijuana in pregnancy, you probably aren’t the type to go cold turkey on cigarettes and alcohol either. You might be a different age or income level than moms who quit toking or who never started. And then there’s the question of the weed itself.


When those two long-term studies started, marijuana that you would buy on the street was about 3 percent THC. Today’s cannabis is stronger: 8 percent or more for similar stuff, and then more concentrated products like hashish can be 20 to 30 percent THC. So it’s possible that using marijuana during pregnancy today is more harmful than it was back in the 1980s.

There’s also a lot of variety in what it means to use marijuana. How often do you use it? Do you smoke it, vape it, or eat it? How far along are you in your pregnancy? (Different parts of the brain and nervous system mature at different times. Brain development actually isn’t complete until you’re in your twenties, which means teenagers probably shouldn’t smoke pot, either.)


Marijuana also contains dozens of psychoactive substances, of which THC just happens to be the best studied. We know very little about what the others do. Cannabidiol, or CBD, is another compound that’s credited with a lot of the drug’s medicinal effects, but we don’t know nearly as much about it as we’d like.

Some varieties of cannabis claim to be low in THC and higher in CBD, which sounds like it should be safer, but labels can’t always be trusted. Dr. Larry Wolk, the head of the Colorado public health department, says potency testing isn’t always accurate enough to guarantee exactly what you’re getting. “It’s hard to make any kind of medical statement about [all] marijuana,” he says. “It’s so heterogenous. It’s not like making a medical statement about ibuprofen.”


One last caveat on those high-CBD products: Dow-Edwards points out that “there are no studies done in humans with CBD in [fetal] development. None.”

Risk and Benefit

If marijuana has these risks, or even potential risks, in pregnancy, it seems logical not to smoke it. But women do use marijuana in about 5 percent of pregnancies. Many use it to manage nausea. And anyone living through the months of hell known as hyperemesis gravidarum has a serious risk/benefit calculation to make. Here’s one woman describing her situation to Vice:

The medical term for what I suffered is “hyperemesis gravidarum,” which is a fancy way to say I was throwing up so much that it was a danger to my health and the health of my baby. Marijuana did help. Immensely. I don’t think I would have made it through without cannabis.


That said, excessive marijuana use can also cause its own hyperemesis syndrome, so more pot doesn’t always mean less nausea.

If you have to choose between potentially damaging your future kid’s IQ or attention versus maybe not making it through the pregnancy, that’s a difficult decision that you should work through with a doctor or provider you trust. But Dow-Edwards says she has met people who say their doctor told them that using marijuana is safer than drinking. “That’s just not true,” she says. So it’s important to make sure your provider understands the benefits and the risks.


What We Still Don’t Know

We don’t know if the effects of marijuana in the two long-term studies would be any different with today’s weed.


We don’t know if there are other effects of marijuana that just haven’t been studied yet. It’s hard to get permission to do human studies, and would be downright unethical to compare marijuana against a placebo in pregnant women.

The Colorado report points out that since marijuana was illegal everywhere in the US until 1996, most studies were looking for harms and not positioned to carefully suss out benefits or tradeoffs. “This...introduces both funding bias and publication bias into the body of literature related to marijuana use,” the report notes.


We can’t go back in time to re-start long term studies, but we can support better studies now. Dr. Wolk hopes that eventually, Colorado’s and other states’ health departments will keep careful records of who uses marijuana in pregnancy and what complications they or their children have. So far, that’s not the case.

Finally, we need more data on everything, especially on CBD and on the 80+ other active components in marijuana besides THC. We also don’t even know all the questions we should be asking. Dow-Edwards points out that five years ago we didn’t know whether smoking marijuana can cause lung cancer; now we’re pretty sure it does. There’s still a lot we don’t know.