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.