Marijuana Use and Pregnancy: Is It Safe?


  • 2.8% of pregnant women say they use marijuana
  • Chemicals in marijuana can cross the placenta and may contaminate breast milk
  • The endocannabinoid system plays an important role in fetal development
  • Studies show that marijuana use does not lead to major birth abnormalities
  • Marijuana use is suspected to cause cognitive impairments later in life, but studies are inconclusive
  • Marijuana use during pregnancy will not affect a child’s overall intelligence or IQ score – It’s no secret that using alcohol or tobacco while pregnant can have a devastating impact on a newborn’s health. But does marijuana fall under the same category as these harmful substances?

While you might be hard-pressed to find a doctor that supports the use of marijuana during pregnancy, evidence of any negative effect remains scarce.

Still, it is essential for expectant mothers to review the available information before making their own decision on this controversial subject.

Who Uses Marijuana?

If you happen to be pregnant and have yet to give up marijuana, you’re not alone. Cannabis tops the list as the most frequently used illicit drug among expectant mothers in Canada and the United States.

According to the 2005 National Survey on Drug Use and Health, 2.8% of pregnant women in the U.S. reported using marijuana at some point in the previous month.

Even still, studies suggest that the rate of marijuana use during pregnancy could be much higher, ranging from 10-16% in middle-class samples to 23-30% among lower income populations.

Marijuana and the Fetus

Like most substances carried by the blood stream, chemicals found in marijuana are believed to cross the placenta when ingested during pregnancy. Marijuana compounds such as THC can also be found in breast milk, meaning that cannabis use can affect a child both before and after birth.

On the other hand, studies show that the endocannabinoid system – the body’s natural cannabinoid system – is highly active during fetal development. In fact, studies have even found endocannabinoids (i.e. 2-arachidonoylglycerol) to be present in breast milk, implying that marijuana compounds may be of benefit to a newborn child.

Cannabinoids are believed to contribute to fetal development in the following ways:

  • Implanting of the embryo to the wall of the uterus
  • Neural development (guiding of newly formed brain cells)
  • Protection from brain cell loss
  • Initiation of milk suckling

Thus, marijuana exposure during fetal development would appear to have a wide-ranging impact on a newborn’s health. However, whether these effects are beneficial or harmful is still unclear.

Effects on Birth Outcome

Decades of research have failed to identify a link between marijuana use during pregnancy and an increased risk of premature birth, miscarriage or major physical abnormalities (Canadian Centre on Substance Abuse, 2009). On the other hand, numerous studies point to maternal marijuana use as having harmless, or even beneficial, effects on birth outcome.

One of the more prominent of these studies was published in 1991 through the collaborative efforts of researchers at the Miami Children’s Hospital (Florida), the Princess Margaret Hospital (Jamaica) and the University of Massachusetts.

The study – led by Dr. Melanie C. Dreher – followed 59 Jamaican children for 5 years following birth and fail to identify any negative difference in development between children of mothers who used cannabis and those who did not. On the other hand, the study revealed a beneficial effect of maternal marijuana use at 30 days old, when the infants scored higher on measures of nervous system stability and reflexes.

Interestingly, when marijuana use was further separated according to frequency, the study found that children of heavy-marijuana-using mothers (>21 marijuana joints per week) also scored higher on a variety of auditory, visual and tactile response measures at 30 days of age in comparison to children of non-users. Furthermore, the heavy-marijuana-use group exhibited higher degrees of alertness, less irritability and fewer startles and tremors than the non-use group.

Likewise, a study published in 1989 found no relationship between moderate levels of marijuana use during pregnancy and abnormal infant behavior on the second day of life among 373 mothers and their newborns.

Effects on Birth Weight

A reduction in birth weight is a commonly cited risk of marijuana use during pregnancy, which seems to be independently supported by a number of studies.

However, a comprehensive analysis of 10 studies, published in 1997, concluded that evidence of marijuana use causing low birth weight is inadequate.

Effects on Cognitive Function

Despite having a minimal impact on birth outcome, marijuana use during pregnancy is believed to have significant, yet subtle effects on cognitive function later in life.

Available evidence comes from 2 large-scale studies – the Ottawa Prenatal Prospective Study (OPPS) and the Maternal Health Practices and Child Development Project in Pittsburgh (MHPCD). The OPPS study began in 1978 and followed 700 middle-class pregnant women residing in Ottawa, Canada from pregnancy till birth, after which 180 women and their children continued to be monitored until adulthood. The MHPCD study began in 1982 and followed 763 newborns until late childhood.

Early assessments of the OPPS children revealed no significant impact of maternal marijuana use on motor, language and cognitive development at ages 1, 2 and 3. On the other hand, the MHPCD study identified a relationship between the use of 1 or more joints per day in the third trimester of pregnancy and a decrease in mental scores at 9 months of age. However, this relationship seemed to disappear at 18 months.

Similarly, later follow-ups revealed significant, yet inconsistent relationships between marijuana use during pregnancy and various cognitive deficits, including poor performance on verbal, memory, visual and attention-related tasks. Still, these relationships seemed to be only identifiable at certain ages, suggesting an overall lack of significance from a scientific standpoint.

For instance, the OPPS study found that children of marijuana-using mothers performed worse on verbal and memory tests at age 4, but when the same tests were given at ages 5 and 6, performance was equal among the marijuana-use and non-use groups. Likewise, the OPPS study identified lower scores of attention in children of marijuana-using mothers at ages 6 and 13-16, but none at ages 9-12, whereas the MHPCD study showed higher scores of attention at age 6 (Richardson et al., 2002).

Based on the contradictory nature of these findings, it is impossible to say for certain whether marijuana use during pregnancy has a negative impact on long-term cognitive performance. However, researchers involved with both studies have suggested the discrepancies to be a result of environmental factors or weaknesses in research methods.

Ultimately, with a lack of replicable results, the only conclusion that these studies seem to give is that there are many factors involved with cognitive development, in which marijuana appears to plays a minor part, if any at all.

Effects on IQ

According to the results of the OPPS study, marijuana use during pregnancy does not appear to have a negative impact on a child’s overall intelligence and IQ scores. These findings were consistent throughout the entire follow-up period – from ages 5 to 16.

Once again, this suggests that evidence of a link between cannabis use during pregnancy and cognitive impairments is weak, since overall intelligence seems to be unaffected.

Conclusion: Parenting Matters More

Despite the assortment of benefits and risks that studies have linked to cannabis use, the environment in which a child is raised seems to be a common underlying factor that is too often overlooked in this context.

Indeed, many of the studies highlighted above gave mention to the importance of parenting in the outcome of child-rearing, even as a possible explanation for their findings.

For instance, the authors of the Jamaican study suggest that the benefits observed in the marijuana-use group may reflect a greater maternal competence of Jamaican marijuana-using mothers, since, unlike in the U.S., cannabis is viewed as a wholesome substance with users tending to possess higher levels of education, independence and resources.

Furthermore, the authors of the OPPS study made note of the differences in early childhood education between the marijuana-use and non-use groups, suggesting that preschool attendance could have been the reason why some of their results were so hard to explain.

Indeed, while the effects of marijuana use during pregnancy remain inconclusive, the role that parenting plays above all in shaping a child’s later years has never been up for debate

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