The offspring of lab rats that were exposed to marijuana smoke during pregnancy took longer to learn and comprehend tasks than rats whose mothers weren't exposed to THC, researchers found.

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At Washington State University, researchers placed pregnant rats in a small transparent chamber, and 60 times a day, for 2 minutes at a time, the moms-to-be got hit with a blast of vaporized cannabis extract.

Photographs show the white haze, sometimes shooting right at the nostrils of the curious animals, sometimes engulfing their tiny heads. The female rats began getting stoned during the week of their mating period, and then for the 21 days of gestation.

The results were another warning for mothers-to-be who like to light up. The offspring of the rats that ingested marijuana during pregnancy showed slowed development. Or, in layman’s terms, “It was like something wasn’t clicking with them,” explains Ryan McLaughlin, an assistant professor in WSU’s Department of Integrative Physiology and Neuroscience.

Rat offspring exposed to THC — tetrahydrocannabinol, the chief psychoactive ingredient in cannabis — and others that were not had to learn how to press one of two levers to get a sugar pellet reward. The correct lever was the one nearest a light.

But then, the next day, there was a change. The rats had to press a right or left lever to get a reward, not the one closest to the light.

The offspring of rats that never inhaled only took 50 to 70 attempts to figure that out and get 10 correct in a row, says McLaughlin.

The THC-exposed offspring had trouble adjusting. A lot more trouble adjusting.

Says McLaughlin, that when the THC-exposed offspring had to deal with pressing either lever giving them a reward, “They had a deficit in flexibility in their ability to update strategies. They were significantly impaired.”

They needed 100 to 120 attempts, twice as many, to get 10 correct in a row.

How this research translates to people is that rats share with us 90 percent of their genome, an organism’s complete set of DNA. Nearly all disease-linked human genes have equivalent genes within the rat genome.

Although only a small percentage of pregnant women say they smoked pot in the past month, their use has dramatically increased, according to a 2017 study in the Journal of the American Medical Association.

Nationally, it has jumped by 62 percent from 2002 to 2014, to 4 percent of total pregnant women.

In Washington, the discharge papers for pregnant women staying at hospitals — presumably for delivery but perhaps other reasons — indicate whether they had used drugs. The number of moms who said they had used marijuana but not any other drugs has “been steadily rising,” says a 2016 report by the state’s Health Services Research Project.

Between 2011 and 2014, that number jumped by 24 percent to a high of 8 per every 1,000 cases, says the report. Marijuana was legalized in Washington state in November 2012.


The highest rates were in southwest Washington, in rural counties like Pacific and Cowlitz counties; and in northeast Washington, places such as Ferry and Pend Oreille counties.

Seattle and the north Puget Sound counties were among the lowest.   

“Now that it’s legal, use of marijuana for medical purposes is commonly accepted,” says McLaughlin. “Yet we don’t know the long-term effects of cannabis use, especially in developing brains.”  

An August story in Kaiser Health News tells of online communities of  groups like “Ganja Mommies” and “CannaMoms.”

“The chats are filled with women asking not whether marijuana could be harmful, but rather whether smoking marijuana could put them at risk of involvement from Child Protective Services,” according to the story.

A posting: “I live in Georgia. … I’m only 5 weeks but I plan to keep smoking since there’s no evidence of it being harmful. Has anyone given birth here without being tested?”

We should be flooded with warnings for pregnant women about pot use, said Susan Astley, a University of Washington epidemiology professor.

By state law, marijuana packaging has to have attached, or the customer has to be given, a warning that includes, “Should not be used by women that are pregnant or breast feeding.”

But Astley says that’s not enough.

With alcohol and pregnancy, “It took 30, 40 years of education,” she says. “And now here we go with marijuana.”

She wants a publicity push about pot and pregnancy like there was with alcohol and pregnancy.

One such alcohol warning poster shows a drawing of a pregnant woman drinking from a bottle, and the booze going directly to the baby. Another shows a fetus drinking a glass of red wine. The headline says, “Say NO to alcohol while pregnant.”

Astley’s area of expertise is fetal-alcohol disorders, but in 1990 she did one of the early studies on women who use marijuana and breast-feed.

She concluded it was “associated with a decrease in infant motor development at one year of age.”

That meant at age 1, a THC-exposed infant was lagging and only at the six-month stage in crawling or beginning to sit up.

“THC likes to concentrate in fat, and breast milk is very high in fat,” says Astley.

McLaughlin says he wants to continue with further refinements to the study.

The first time the rats are exposed to THC vapors, he says, they “tend to make little runs” in the enclosure. “They’re very afraid of new things.”

But then you see some rats directly poking their noses into the cloud.

“It seems like they’re like intrinsically interested in vapor,” says McLaughlin.