Prenatal cannabis exposure is linked to a higher risk of several types of structural birth defects, according to a growing body of research that includes large cohort studies and meta-analyses. The picture is more complicated than a simple yes-or-no, though. THC crosses the placenta and reaches the developing fetus, where it can interfere with signaling systems that guide organ formation and brain wiring. But the size of the risk, the specific defects involved, and the difficulty of separating cannabis from other factors like tobacco use all make this a field where confident, clean answers are still hard to come by.
What the Meta-Analyses Show About Structural Defects
The strongest signal in the research comes from a cumulative meta-analysis published in 2024 that pooled data across multiple studies and found elevated odds of birth defects across several organ systems. Babies exposed to cannabis prenatally showed roughly double the odds of cardiovascular or heart defects, gastrointestinal defects, central nervous system defects, and genitourinary defects compared to unexposed babies. A broader “any birth defect” category also showed a smaller but statistically meaningful increase in risk.1PubMed. The association between prenatal cannabis use and congenital birth defects in offspring: A cumulative meta-analysis
Not every study agrees, however. A large 2025 study of nearly 364,000 infants in which about 6% were exposed to prenatal cannabis found no statistically significant associations between maternal cannabis use and most categories of major structural birth defects.2PubMed Central. Maternal Prenatal Cannabis Use and Major Structural Birth Defects This kind of contradictory result is common in this area. Different studies rely on different methods of measuring exposure (self-report versus biological testing), different definitions of what counts as a birth defect, and different ways of adjusting for confounding factors like tobacco and alcohol. The meta-analytic evidence leans toward a real association, but the absolute risk to any individual pregnancy remains low in most studies.
The Gastroschisis Connection
One specific defect keeps showing up in the literature with more consistency than most: gastroschisis, a condition where a baby is born with intestines protruding through an opening in the abdominal wall. Multiple reviews have flagged this as the structural anomaly most reliably associated with prenatal cannabis exposure.3PubMed Central. Update on the developmental consequences of cannabis use during pregnancy and lactation
A California study covering 2007 to 2019 found a positive association between cannabis use disorder during pregnancy and gastroschisis.4PubMed Central. Prenatal cannabis use disorder and gastroschisis in California, 2007-19 A separate study looking specifically at the more severe “complex” form of gastroschisis reported that cannabis use was more common among mothers of affected babies even after adjusting for exposure to smoking, narcotics, and alcohol, with adjusted odds roughly two and a half times higher.5PubMed. Maternal risk factors associated with complex gastroschisis: Cannabis exposure and recurrent urinary tract infections may be modifiable targets
Gastroschisis is still rare overall, affecting roughly a few in every 10,000 births. But the consistency of its association with cannabis across studies, and the biological plausibility behind it, make it one of the more credible specific risks in this research area.
How THC Reaches and Affects the Fetus
THC, the main psychoactive component in cannabis, crosses the placenta. That much is well established. Cannabinoid receptors have been found in fetal brain tissue and in the placenta itself.6PubMed Central. Marijuana Use in Pregnancy: A Review CBD also crosses the placenta, meaning the effects of cannabis exposure during pregnancy are not limited to THC alone. Both compounds can affect the fetus directly and can alter how the placenta functions, potentially disrupting nutrient and oxygen delivery.7PubMed. Cannabinoids and the placenta: Receptors, signaling and outcomes
Research using a perfused human placenta model estimated that fetal blood THC concentrations reach roughly a quarter to a third of maternal levels at steady state. The placenta has transporter mechanisms that partially limit how much THC gets through, but they do not block it entirely.8PubMed. Understanding the Mechanism and Extent of Transplacental Transfer of (-)-∆9 -Tetrahydrocannabinol (THC) in the Perfused Human Placenta to Predict In Vivo Fetal THC Exposure
The reason this matters for birth defects goes beyond simple toxicity. The body’s own endocannabinoid system, which THC mimics, is active from the earliest embryonic stages and plays a role in guiding how embryonic cells differentiate, how axons migrate in the developing brain, and how the embryo implants in the uterus. Disrupting these tightly regulated signals at the wrong moment could theoretically cause structural problems.9PubMed. The endocannabinoid system during development: emphasis on perinatal events and delayed effects10PubMed. Multiple roles for the endocannabinoid system during the earliest stages of life: pre- and postnatal development
THC as a “Conditional Teratogen”
One of the more striking findings in recent years comes from mouse research showing that THC can act as what researchers call a “conditional teratogen.” In this study, THC interfered with the Hedgehog signaling pathway, which is critical for early patterning of the brain and neural tube. In genetically normal mice, THC alone did not cause visible defects. But in mice carrying a mutation that already partially weakened Hedgehog signaling, THC exposure tipped the balance and produced hallmark defects including holoprosencephaly (a failure of the forebrain to divide properly) and neural tube patterning problems.11PubMed Central. Δ9-Tetrahydrocannabinol inhibits Hedgehog-dependent patterning during development
This “conditional” framework is useful because it could help explain why cannabis does not cause birth defects in every exposed pregnancy. The idea is that THC might only push development off course in fetuses that are already genetically susceptible, carrying subtle variants that reduce their buffering capacity against environmental insults. If true, this would mean population-level studies see a modest average increase in risk, while the actual danger is concentrated in a subset of pregnancies that researchers currently have no way to identify in advance.
Low Birth Weight, Preterm Birth, and Other Perinatal Outcomes
Even setting aside structural birth defects, the evidence on growth-related outcomes is more consistent. Multiple meta-analyses have found that cannabis use during pregnancy is associated with lower birth weight, higher odds of being born small for gestational age, and an increased chance of preterm delivery.
A 2022 meta-analysis in JAMA Network Open reported that marijuana-exposed newborns had roughly double the risk of weighing less than 2,500 grams at birth, along with a mean birth weight reduction of about 112 grams. These babies also had a higher rate of NICU admission, smaller head circumference, and lower one-minute Apgar scores.12JAMA Network Open. Birth Outcomes of Neonates Exposed to Marijuana in Utero: A Systematic Review and Meta-analysis A separate meta-analysis found a similar pattern, with adjusted odds of about 1.5 for low birth weight, preterm birth, and small-for-gestational-age, and noted a possible dose-response effect: heavier use and continued use into later trimesters appeared to carry higher odds.13PubMed. The Effects of Cannabis Use during Pregnancy on Low Birth Weight and Preterm Birth: A Systematic Review and Meta-analysis
An earlier meta-analysis found cannabis-exposed infants weighed about 109 grams less on average, consistent with the later estimates.14PubMed Central. Prenatal exposure to cannabis and maternal and child health outcomes: a systematic review and meta-analysis These are not dramatic reductions for most pregnancies, but birth weight sits on a continuous risk spectrum: every gram lost at the low end pushes a baby closer to clinical thresholds where complications become more common.
The Tobacco Problem and Why It Matters
One of the biggest headaches in this research is that many people who use cannabis during pregnancy also smoke tobacco, drink alcohol, or use other substances. Tobacco itself is a well-established cause of low birth weight and has been linked to certain birth defects. Disentangling the two is genuinely difficult.
Studies that have specifically looked at co-use of cannabis and tobacco found the combination carries worse outcomes than either substance alone. One study found that co-use was associated with the highest odds of small head circumference and birth defects, with the co-use group having over three times the odds of a birth defect compared to non-users.15PubMed Central. Prevalence and Associated Birth Outcomes of Co-Use of Cannabis and Tobacco Cigarettes during Pregnancy Another study examining developmental cohorts found that co-exposure to cannabis and tobacco was associated with an estimated 18% reduction in birth weight that could not be explained by earlier delivery, alcohol, other drugs, or socioeconomic factors.16PubMed Central. Additive drug-specific and sex-specific risks associated with co-use of marijuana and tobacco during pregnancy: Evidence from 3 recent developmental cohorts (2003-2015)
Researchers try to control for tobacco statistically, but residual confounding is always a concern. Some skeptics argue that the birth defect associations attributed to cannabis could partly reflect unmeasured tobacco effects or lifestyle factors that cluster with substance use. The meta-analyses that do adjust for tobacco still find elevated risks for cannabis, which strengthens the case for an independent effect, but “strengthens” is not the same as “proves.”
Self-Reporting and the Measurement Problem
Most large studies rely on pregnant women self-reporting their cannabis use, and this is a known weak spot. A scoping review on the validity of self-report measures found low agreement between what women said about their third-trimester cannabis use and what biological testing of their baby’s meconium showed.17PubMed Central. Validity of self-report measures of cannabis use compared to biological samples among women of reproductive age: a scoping review Another study noted that positive maternal oral fluid tests called into question some self-reports of quitting, suggesting underreporting is common, especially as pregnancy progresses and social pressure to deny use increases.18Clinical Chemistry. Identifying Prenatal Cannabis Exposure and Effects of Concurrent Tobacco Exposure on Neonatal Growth
Underreporting means that the “unexposed” groups in many studies likely include some cannabis users, which would dilute the apparent effect. In other words, the true difference between exposed and unexposed babies could be larger than what studies currently report. On the other hand, women who do report use might differ from those who do not in ways that are hard to measure, introducing bias in the other direction. These measurement challenges are a real limitation, and they are one reason this field is not as settled as, say, the evidence on alcohol and fetal alcohol spectrum disorders.
Today’s Cannabis Is Not the Same Product
Most of the landmark cohort studies on prenatal cannabis exposure began decades ago when THC concentrations in typical marijuana were much lower than they are now. This raises the uncomfortable question of whether modern high-potency products carry greater risks than the older research captures.
A recent study that measured daily THC intake among pregnant cannabis users found an average of about 198 milligrams of THC per day. Newborns in this exposed group weighed less and were shorter for their gestational age, and female newborns had notably smaller head circumferences compared to both controls and exposed males.19PubMed Central. High-Potency Prenatal Cannabis Exposure and Birth Outcome Measures That sex-specific finding is intriguing and echoes patterns seen in some animal research, though it needs replication in larger samples.
Complicating matters further, no studies have yet examined whether different methods of consumption, such as smoking versus vaping versus edibles, have distinct effects on fetal outcomes. Vaping has become the second most common method of cannabis use among pregnant women, yet its specific risks remain essentially unknown.20PubMed Central. Women’s Experiences with Nicotine and Cannabis Vaping During Pregnancy and Postpartum
CBD Is Not a Safe Alternative
Many people assume that CBD products, which do not produce a high, are safe during pregnancy. The animal research increasingly suggests otherwise. In rats, CBD exposure during pregnancy resulted in pups that were about 10% smaller, with placentas showing disrupted blood vessel formation and altered expression of glucose transporters. The placental changes point toward reduced nutrient delivery as a mechanism for the growth restriction.21PubMed Central. Cannabidiol Exposure During Rat Pregnancy Leads to Labyrinth-Specific Vascular Defects in the Placenta and Reduced Fetal Growth
Mouse studies have gone further, finding that fetal CBD exposure altered pain sensitivity in adult male offspring and decreased problem-solving behaviors in females, along with measurable changes in the electrical activity of prefrontal cortex neurons.22Molecular Psychiatry. Fetal cannabidiol (CBD) exposure alters thermal pain sensitivity, problem-solving, and prefrontal cortex excitability A reproductive toxicity evaluation of CBD isolate in rats found that at the highest doses tested, there were cases of total litter loss and difficult labor, though developmental effects at lower doses were limited to reduced pup weights.23PubMed. Reproductive and developmental toxicity evaluation of cannabidiol
These are animal studies, and the doses involved are often much higher relative to body weight than what a person would typically consume. But they firmly close the door on the idea that CBD, just because it is non-intoxicating, is automatically safe for a developing fetus.
Neurodevelopmental Effects Beyond Structural Defects
Even when a baby is born without a visible structural anomaly, prenatal cannabis exposure may leave subtler marks on brain development. Both human longitudinal studies and animal research indicate that the endocannabinoid system plays a significant role in shaping brain regions involved in mood, cognition, and reward. Disrupting this system during development does not appear to be benign.24SpringerLink / European Archives of Psychiatry and Clinical Neuroscience. Neurobiological consequences of maternal cannabis on human fetal development and its neuropsychiatric outcome
A systematic review and meta-analysis looking at cognitive and behavioral outcomes found that prenatal cannabis exposure was associated with poorer attention and more externalizing behaviors (things like aggression and impulsivity) in infancy and early childhood. Evidence for effects on other cognitive domains was less clear.25PubMed Central. Birth, cognitive and behavioral effects of intrauterine cannabis exposure in infants and children: A systematic review and meta-analysis Earlier reviews pointed to overlaps with the neurobehavioral profiles seen in children exposed to tobacco and alcohol, including symptoms resembling ADHD and deficits in learning and memory.26PubMed. Maternal smoking, drinking or cannabis use during pregnancy and neurobehavioral and cognitive functioning in human offspring Long-term follow-up data from prospective cohorts suggest these impacts on cognitive function can persist.27PubMed Central. Lasting impacts of prenatal cannabis exposure and the role of endogenous cannabinoids in the developing brain
Importantly, researchers have not identified a unique “cannabis-exposed” pattern the way fetal alcohol syndrome has distinctive facial features and developmental characteristics. The effects of prenatal cannabis look more like a statistical nudge toward attention and behavioral difficulties rather than a recognizable syndrome.
Why Some Pregnant People Use Cannabis Anyway
Morning sickness is the main driver. In one survey, about two-thirds of respondents who had experienced nausea or vomiting during pregnancy had used cannabis to treat it, and over 90% rated it as effective or extremely effective.28PubMed. Survey of medicinal cannabis use among childbearing women: patterns of its use in pregnancy and retroactive self-assessment of its efficacy against ‘morning sickness’ A Michigan study found that women with more severe morning sickness had higher odds of using cannabis, with about 14% of the sample reporting use.29PubMed Central. Nausea and vomiting of pregnancy and prenatal cannabis use in a Michigan sample A Hawaii study found a similar pattern, with women experiencing severe nausea being significantly more likely to report marijuana use.30PubMed Central. Marijuana use and maternal experiences of severe nausea during pregnancy in Hawai’i
This puts clinicians in a tricky spot. Severe morning sickness (hyperemesis gravidarum at its worst) is not a trivial condition; it can cause dehydration, weight loss, and hospitalization. When prescription anti-nausea medications fail or are not offered, some women turn to cannabis because it works quickly and they perceive it as “natural.” The perception that cannabis is safer than pharmaceutical options is widespread but not well supported by the evidence on fetal outcomes.
Paternal Cannabis Use and Epigenetics
An emerging area of research asks whether the father’s cannabis use before conception could also matter. A 20-year intergenerational cohort study found effects of preconception cannabis use on offspring birth outcomes and discussed possible mechanisms. Animal studies have shown that cannabis exposure can alter epigenetic marks on sperm, including changes to DNA methylation and histone modifications. These altered marks could theoretically transmit effects to the next generation. Recent evidence has also suggested that cannabinoid exposure affects methylation patterns in human sperm.31Scientific Reports. Cannabis and tobacco use prior to pregnancy and subsequent offspring birth outcomes: a 20-year intergenerational prospective cohort study
This is still early-stage research, and no one would call it definitive. But it raises the possibility that the conversation about cannabis and reproductive risk should not be limited to the pregnant person alone.
Cannabis in Breast Milk
The question of fetal exposure does not end at delivery for breastfeeding parents who use cannabis. THC transfers into breast milk and is absorbed by the nursing infant.32Neurotoxicology and Teratology. Maternal marijuana use during lactation and infant development at one year A study that measured cannabinoid levels in breast milk found detectable THC along with several metabolites. The milk of cannabis-using mothers also showed altered levels of lactose and lower levels of secretory IgA, an immune protein that helps protect infants from infections.33Pediatric Research. Cannabis use during lactation may alter the composition of human breast milk THC is fat-soluble, which means it accumulates in the fatty component of breast milk and can build up with ongoing consumption. Given that infant brains are still rapidly developing during the breastfeeding period, this represents a continued route of exposure even after birth.