Will smoking marijuana during pregnancy harm the child?

The question

My daughter is pregnant and she has been smoking some marijuana to reduce nausea and vomiting from morning sickness. She says marijuana is a natural product and it won't hurt her unborn child. Is she right?

The answer

There is no evidence that marijuana causes obvious birth defects. And this fact may be contributing to the impression that it's safe to use during pregnancy.

But "it is not a benign substance," warns Dr. Erin Lurie, a fellow in addictions medicine at the Centre for Addiction and Mental Health in Toronto.

A growing body of research suggests that exposure to marijuana in the womb has other, more subtle, effects – especially on the developing brain. In a nutshell, it may increase the baby's risk of learning difficulties, impulsiveness and inattention, as well as other behavioural and mental-health problems later in life.

Separate research teams in Ottawa and Pittsburgh have carried out the longest-running studies on the children of women who smoked pot during pregnancy.

The Ottawa study began in 1978, with the recruitment of about 600 pregnant women mostly from middle-class backgrounds. Some of the women had smoked marijuana; others smoked tobacco; still others used marijuana and tobacco; the rest abstained from both substances. The amount of marijuana consumed, and the duration of their use, varied from woman to woman. Their offspring have undergone extensive testing at regular intervals into adulthood. A select sample from the offspring continues to be followed up.

The Pittsburgh study started in 1982, and also included about 600 mothers, although most were from low-income backgrounds.

Both these studies, plus other research, point to the same conclusions. "I think the similar results lend a lot of credence to our findings," says the lead researcher of the Ottawa study, Peter Fried, professor emeritus in the department of psychology at Carleton University.

In particular, the studies indicate that marijuana affects certain aspects of executive functioning, which primarily take place in the brain's prefrontal cortex.

Executive functioning "enables you to make decisions and anticipate the consequences of your actions," explains Fried.

Marijuana also impairs attention and visual problem solving – for example, doing something as ordinary as a jigsaw puzzle.

As well, "the lack of sustained attention usually means they can't inhibit themselves from doing other things." This can translate into impulsive behaviour.

Fried says some of the effects of marijuana exposure are not immediately apparent in newborns. It wasn't until the children were four years old that cognitive testing revealed significant differences in the executive functioning of offspring of mothers who smoked dope, compared to kids of abstaining moms.

Researchers have a limited ability to measure executive functioning in very young children, he explains. "If there were better tests, the differences might have been spotted sooner."

In recent years, the Ottawa researchers started doing functional MRI brain scans on the study participants. These high-tech fMRI tests reveal that offspring exposed to marijuana prenatally seem to "utilize different parts of the brain" for performing certain tasks in comparison to unexposed offspring.

"It's almost like the brain corrected itself" for the changes brought about by marijuana exposure, says Fried. "But interestingly, the length of time it took the subjects to respond [to a task] was somewhat longer."

One thing is clear from the research: "The more the woman smokes up, the greater the likelihood there will be some effect on the offspring," says Fried.

He emphasizes that his study was launched in the late 1970s when marijuana was far less potent than it is today. The concentration of tetrahydrocannabinol (THC) – the ingredient that makes people feel high – is much greater now. So, it's possible that studies based on exposure decades ago "may represent a somewhat conservative picture of the impact of marijuana use on a developing fetus," says Fried.

The effects may also vary depending on how the marijuana is consumed – whether it is smoked or consumed as an oil or edible. Smoke from a marijuana cigarette contains five times more carbon monoxide than the smoke from a tobacco cigarette, which can affect the amount of oxygen reaching the fetus.

Many public-health experts are concerned that more pregnant women will use marijuana once Canada legalizes recreational pot next year.

There are no reliable statistics on how many Canadians currently smoke marijuana during pregnancy. U.S. studies have found that four to six per cent of pregnant women acknowledge using it, although that figure is likely an underestimate, says Lurie.

The move to legalization seems to be reducing the stigma associated with marijuana consumption. "People just don't even consider it to be a drug," says Lurie.

Some health-care providers have noticed that more and more women are willing to discuss their use of cannabis during pregnancy.

"They are letting their guard down and feel safer disclosing its use," says Kelly Polci, a social worker in the Women and Babies Program at Sunnybrook Health Sciences Centre in Toronto.

In some cases, women struggling with morning sickness are turning to marijuana – a remedy that is promoted on some websites.

"There is a lot of messaging on the Internet saying that marijuana is safe to use during pregnancy, while it is actually not," says Dr. Maya Nader, a family physician at St. Michael's Hospital in Toronto.

"Women need to know that health-care providers can recommend other morning-sickness treatments that don't carry the risks of marijuana."

What's more, women should also be aware that breastfeeding and marijuana are a bad combination. THC is stored in fat tissue and can remain there for up to two months, gradually being released into breast milk. That means some children can get a dose of THC from breastfeeding – even after their mothers have stopped smoking marijuana, says Nader.

Abstinence is the best approach. But if a pregnant woman or new mom is having trouble quitting, she should talk to her doctor in order to reduce the potential harm to her offspring, advises Lurie.

Paul Taylor is a patient navigation advisor at Sunnybrook Health Sciences Centre. He is a former health editor of The Globe and Mail. You can find him on Twitter @epaultaylor and online at Sunnybrook's Your Health Matters