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Approximately four per cent of North American women use . With more than  and the , this means that over 166,000 babies are exposed prenatally every year. Cannabis use in pregnancy rose by 62 per cent between 2002 and 2014.

Now that much of the U.S. has some form of legalized cannabis, and recreational use is , consumption will likely keep increasing. This raises important concerns about the safety of prenatal cannabis exposure for the developing brain.

National obstetric societies in North America recommend . However, recent media coverage has . It has even been suggested that recommendations to avoid its use . Social acceptability and the public perception of .

As researchers in neuroscience, child development and obstetrics, we study the effects of prenatal substance exposures on infant and child outcomes. We maintain that, despite media coverage suggesting otherwise, there remains no known safe amount of prenatal cannabis exposure.

The fetal brain , making it more vulnerable to substances than at any other time in life. Cannabis metabolites cross the placenta and can remain in the fetus for days. Cannabis can disrupt cell growth and migration, .

Effects on birth outcomes

The negative effects of prenatal cannabis exposure on birth outcomes are clear. Babies exposed to cannabis in the uterus are more likely to be , be Ìý²¹²Ô»åÌý than babies not exposed to cannabis. Mothers who use cannabis in pregnancy are more likely to have anemia and difficulty breastfeeding than mothers who do not use cannabis in pregnancy.

Cannabis in pregnancy may also interact with other substances. Use of  was associated with increased risks for both mothers and infants, including pre-eclampsia, pre-term birth and lower birthweight. A study  in mice and zebrafish found that even at low doses, incidence of birth defects was higher with combined exposure than with either substance alone.

Effects on learning

The effects of pre-natal cannabis exposure on children’s learning are less clear. Findings have varied, which may contribute to recent mixed messaging.  on this topic, we noted that some studies report negative effects of prenatal cannabis use on children’s learning and others find no substantial effects. However, two relevant points are important to consider when weighing the evidence.

First, many existing studies are limited by inaccurate measurement of cannabis exposure. They also do not control for potential confounders (examples of confounders include income, education or use of other substances). That means that other factors may influence the results and hide significant effects.

Second, and more importantly, a lack of evidence for harm is not the same as evidence of safety. Large, well-designed studies are needed to truly understand the long-term effects of prenatal cannabis exposure on learning in children. Encouragingly, new research is emerging. A recent study looked at more than 500,000 babies born between 2007 and 2012. Children prenatally exposed to cannabis were  than children not prenatally exposed to cannabis.

Safety versus lack of evidence of harm

Not only is prenatal cannabis use rising, but Ìý³ó²¹²õÌý. Other methods of consumption,  and topical products are also now more accessible. Most studies examining the impacts of prenatal cannabis exposure on children’s learning collected data in the 1970s and 1980s. These studies therefore do not represent patterns of prenatal cannabis use today.

Studies that accurately measure prenatal cannabis use and properly control for other variables are urgently needed. High quality research is the only way to ensure that recommendations on prenatal use are strongly supported by scientific evidence.

The associations between prenatal cannabis use and learning are not yet fully understood. However, it is clear that infants exposed to cannabis in utero are at higher risk of pre-term birth and being small for gestational age. Therefore, we support current recommendations against cannabis use in pregnancy as accurate and responsible. Pregnant women should be advised about the potential negative effects of cannabis use and offered support to reduce or stop use wherever possible.

We look forward to more research in this area, so potential links between in-utero cannabis exposure and learning and behaviour can be clarified. Accurate, recent data is the best way to inform appropriate recommendations on prenatal cannabis use.