Unborn children whose mother ingested THC during pregnancy could possibly have their brain development impacted negatively, but signs of the cognitive impairment may not show up until later in life. A couple of studies that began almost 40 years ago tracked children and found some had learning disabilities.
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.
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.
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.