Cannabis use during pregnancy is a growing public health concern, especially as legalization increases its availability. The medical consensus strongly advises against using any form of cannabis, including products containing THC or CBD, while pregnant or attempting to conceive. This recommendation is based on evidence that components of the cannabis plant can cross the placental barrier and affect the developing fetus. A rising number of women report using cannabis during pregnancy, often to alleviate symptoms like morning sickness. This trend is concerning because the potency of THC in modern products is significantly higher than in the past, potentially increasing the risk of adverse outcomes.
The Biological Pathway to the Fetus
THC, the primary psychoactive compound in cannabis, is highly fat-soluble, allowing it to pass easily through biological membranes. Once absorbed into the mother’s bloodstream, THC readily crosses the placenta, the organ responsible for nutrient and gas exchange between mother and fetus. The placenta does not effectively filter out THC, allowing it to enter the fetal circulation and accumulate in the baby’s fat-rich tissues, including the developing brain. This transfer is sufficient to cause biological effects.
The fetus has an active endocannabinoid system (ECS) very early in gestation, possibly as soon as five weeks after conception. The ECS is an internal signaling network that regulates processes like the proper migration and differentiation of brain cells. When external cannabinoids like THC enter the fetal environment, they bind to the same receptors, disrupting the delicate balance of the ECS. This interference is the main mechanism by which prenatal cannabis exposure negatively influences fetal development and placental function.
Impacts on Fetal Growth and Birth Outcomes
Prenatal cannabis exposure is consistently associated with immediate consequences on fetal growth and the timing of birth. A common finding is an increased risk of Intrauterine Growth Restriction (IUGR), meaning the baby does not grow to its full potential in the womb. This growth restriction elevates the likelihood of the baby being born Small for Gestational Age (SGA) or with a low birth weight (LBW). Mothers who use cannabis during pregnancy are more than twice as likely to have a low-birth-weight baby compared to those who abstain.
The association between cannabis use and low birth weight is often reported as a reduction of approximately 80 to 110 grams in the baby’s average weight at birth. This reduction is observed even after accounting for other factors that influence birth size, such as tobacco use or socioeconomic status. Cannabis use during gestation is also linked to a higher risk of preterm birth (delivery before 37 completed weeks). Pregnant women who use cannabis face a nearly 1.5 times higher risk of delivering prematurely.
Prematurity and low birth weight are the main risk factors for infant illness and mortality. Infants exposed to cannabis in utero are more than twice as likely to require admission to the Neonatal Intensive Care Unit (NICU) after birth. Heavy or continued cannabis use, especially into the second and third trimesters, is associated with the highest odds of these adverse birth outcomes. THC can compromise the function of the placenta itself, reducing the transfer of oxygen and nutrients to the fetus, which contributes to reduced growth.
Childhood Neurodevelopmental Considerations
Prenatal cannabis exposure is associated with developmental differences that can persist throughout childhood and into adolescence. Disruption of the fetal endocannabinoid system, which guides the formation of brain circuits, leads to lasting changes in neurodevelopment. Research points toward specific cognitive deficits, particularly in domains such as attention, memory, and executive function. Short-term memory and verbal reasoning skills are sometimes negatively affected in exposed children, especially when maternal use was heavy during the first and second trimesters.
These neurodevelopmental effects are supported by findings of altered brain structure and function in exposed children. Changes have been noted in brain regions like the corpus callosum and the prefrontal cortex, which are responsible for complex decision-making and cognitive control. These structural differences may underlie observed difficulties with planning, integration, and judgment skills.
Behavioral issues are another area of concern, with studies indicating an increased risk for impulsivity, hyperactivity, and symptoms related to Attention Deficit Hyperactivity Disorder (ADHD). Longitudinal studies suggest these behavioral and cognitive challenges can extend into adolescence. Early life exposure may also increase future vulnerability to mental health conditions, with some cohorts showing higher rates of depressive symptoms and an increased risk of psychosis in young adulthood.
Transfer Through Breast Milk
The concern regarding cannabis exposure does not end at delivery, as THC readily transfers into breast milk during the postpartum period. Because THC is highly lipophilic (fat-soluble), it has a strong affinity for the fat content in human milk. This allows THC to concentrate in the milk, sometimes reaching levels up to eight times higher than in the mother’s bloodstream. Breast milk acts as a reservoir, trapping the THC and leading to prolonged exposure for the nursing infant.
When an infant ingests THC through breast milk, it enters a still-developing brain undergoing rapid growth and maturation. Caution is warranted due to the potential for the drug to interfere with the infant’s neurodevelopment, though research on long-term effects is limited. Observations in exposed infants have included subtle effects such as lethargy, less frequent feeding, and shorter feeding sessions. Given the potential for THC to affect the infant’s brain and the lack of comprehensive safety data, abstinence from cannabis is strongly advised throughout lactation.