Cannabis in Pregnancy: What Two Large Research Reviews Can — and Cannot — Tell Families

A plain-language guide to a 2025 analysis of nearly eight million pregnancies and a 2026 review of how exposed children develop

Two recent research reviews looked at what happens when cannabis is used in pregnancy. Together they suggest that exposed babies are, on average, slightly smaller and slightly more likely to be born early or need intensive care, and that as children grow, the differences that appear are in attention, behaviour and self-control rather than intelligence or language. The effects on any one child are small, and researchers cannot yet prove cannabis itself is the cause.

Why this question is hard, and why it matters

Cannabis has changed faster than the research about it. It is now legal in many places, sold in far stronger forms than a generation ago, and widely believed to be gentler than prescription medicines. Many people use it in pregnancy for real reasons — severe nausea, poor sleep, anxiety, pain — and often after being told to avoid other treatments. Doctors and midwives are then asked a fair question they find difficult to answer precisely: what does this actually do to my baby?

Surveys covering 2021 to 2023 show that cannabis use among pregnant people has continued to rise [1]. Medical bodies have not changed their advice: the American College of Obstetricians and Gynecologists reaffirmed in 2025 that cannabis should be stopped during pregnancy and breastfeeding [2]. But "stop" is easier to say than to explain, and families reasonably want to know what the evidence behind it looks like. Two reviews published eleven months apart now give the clearest available answer. One, from a research institute in Mesa, Arizona, combined 51 studies of what happens at birth [3]. The other, from the University of Porto in Portugal, gathered 72 studies of how exposed children develop from birth to age 18 [4].

What families and clinicians used to face

For decades, this was one of the least well-served questions in pregnancy care. Almost everything known came from three studies begun in the 1980s, which followed a few hundred children into their teenage years. Those studies found small differences in attention and self-control, but nearly every exposed pregnancy in them also involved cigarettes, so no one could tell which substance was responsible. A widely read summary of that era concluded, in effect, that the pattern was real but the explanation was unproven [5] — and that uncomfortable verdict stood for years.

Meanwhile the situation on the ground shifted twice over. Cannabis began to be used deliberately as a treatment for pregnancy symptoms, especially severe nausea and vomiting, and people who used it that way generally reported that it helped [6]. At the same time, products became far more potent, so the children studied in the 1980s were exposed to something chemically weaker than what is on sale today, and even good old research may understate the current picture. Families were, in practice, being given advice built on evidence that no longer quite matched their circumstances. Both new reviews were written to close that gap.

How the studies were done

Neither review put anyone in an experiment; that would be unethical. Both rely on observational research: comparing pregnancies where cannabis was used with pregnancies where it was not, then using statistics to account for the many other ways those groups differ.

The birth-outcomes review searched five research databases through 1 March 2024, screened more than 5,000 studies and kept 51 [3]. Together those studies covered 7,920,383 pregnancies, of which 111,939 involved reported cannabis use. Because the studies varied so much, the researchers combined them using a method designed to allow for that variation, and they graded each study's quality; most were judged fair.

The child-development review searched three databases through 25 October 2025 and included 72 studies published between 1985 and 2025 [4]. The team had originally planned to combine the results into a single number, but the studies measured exposure and development in such different ways that averaging them would have produced a figure that meant nothing. So instead they grouped findings by area of development — thinking, language, movement, attention, behaviour, planning and self-control — and by the child's age. The studies themselves were of good quality: 70 of the 72 scored in the top band of a standard quality checklist.

It is worth knowing why anyone expects an effect at all. Laboratory work using human placentas has shown that THC, the main active ingredient in cannabis, crosses from the mother's blood into the baby's circulation in meaningful amounts [7]. Once there it acts on a natural signalling system that helps guide how brain cells multiply, move into position and connect to one another [8]. So there is a plausible biological route — not proof, but a reason to look carefully.

What was found at birth

The pooled results all point the same way, and all are small in absolute terms [3]. Babies exposed to cannabis weighed on average 178 grams less at birth — roughly six ounces, about the weight of a small apple. Their head circumference was smaller by about a third of a centimetre and their length by just under one centimetre. Pregnancies were shorter by about a day and a half on average.

Turned into risks rather than averages, the picture is: about 70% higher chance of being born under 2,500 grams; about 80% higher chance of being small for gestational age, meaning smaller than expected for the number of weeks; about 40% higher chance of being born before 37 weeks; about 55% higher chance of needing neonatal intensive care; and a raised risk of death around the time of birth, though that last finding was less certain than the others. A slightly higher rate of major birth defects was also seen, but only after one unusual study was set aside.

Percentages like these sound alarming until they are anchored. If an outcome affects 5 in 100 babies normally, a 55% increase means about 8 in 100 — a real change, but one in which the great majority of babies are unaffected. One important limitation applies throughout: only around 39% of the studies had accounted for cigarette smoking, which often goes together with cannabis and independently causes smaller babies. So these figures are probably the largest the true effect could be, not the most accurate estimate of it. Other reviews that adjusted more carefully found the same direction with somewhat smaller numbers [9],[10]. And a study comparing cannabis users who smoked cigarettes with those who did not found lower birth weights with frequent cannabis use either way, suggesting cannabis contributes something of its own [11].

What was found as children grew

Here the message is more specific, and in one sense more reassuring than many parents expect [4]. In studies of general thinking ability, 84% found no difference between exposed and unexposed children once other factors were taken into account. In studies of language, 92% found no difference. In studies of early movement skills, 60% found no difference.

The differences that did show up clustered in one area: the ability to regulate attention, behaviour and impulses. Among studies of behaviour, 73% found more difficulties in exposed children, and that proportion rose with age, reaching 93% of studies done at school age. Among studies of attention, 69% found more difficulties. Among studies of planning and self-control, 70% did. These are the skills that mature slowest in childhood and adolescence, which fits the idea that the brain systems still under construction longest are the ones most sensitive to an early disturbance.

Two further details matter. Children whose mothers used cannabis more heavily, or who continued after knowing they were pregnant, tended to show larger differences — which suggests that stopping, at any point, is likely to help. And the picture was less clear when researchers looked at formal diagnoses such as ADHD in medical records rather than at directly measured attention [12],[13]. Researchers also stress a genuine alternative explanation: some people use cannabis to manage their own attention, anxiety or mood difficulties, and those conditions can run in families for reasons that have nothing to do with pregnancy exposure.

What this means for you and your baby

If you used cannabis before you knew you were pregnant, this evidence does not say your baby has been harmed. Most exposed babies are born healthy and develop normally, and nothing in either review allows anyone to look at one child and attribute a difficulty to the exposure.

If you are pregnant now and using cannabis, the practical message is that stopping is worth doing and that stopping later is still better than not stopping. It is also worth telling your care team, because the point of knowing is to watch your baby's growth and, later, to keep an eye on attention and behaviour as they approach school — not to judge anyone. If nausea, pain, sleep or anxiety is the reason you are using it, say so; those are treatable problems, and a clinician who knows the real reason can offer a real alternative.

What researchers are working on next

The main obstacle now is measurement rather than numbers of participants. Most studies still rely on people remembering and reporting their own use, which is imprecise, and few can measure how much or how strong the product was, or exactly when in pregnancy it was used. Researchers have set out clearly what better studies would look like: verified exposure, measured doses, timing by trimester, and designs — such as comparing brothers and sisters within the same family — that can separate the exposure from everything else families share [14]. Perinatal specialists are also making the case that public understanding of cannabis has drifted well ahead of the evidence, and needs to be brought back into line [15]. Until those better studies arrive, the honest summary is the one above: consistent, small, real, and not yet proven to be caused by cannabis alone. In[Neo]Sight covers related questions about other pregnancy exposures, including how newborn care teams support babies exposed to opioids.

References

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  2. American College of Obstetricians and Gynecologists. Clinical Consensus No. 10: Cannabis Use During Pregnancy and Lactation. Obstet Gynecol. 2025;146(4):600–611. doi:10.1097/AOG.0000000000006053 ↩
  3. Sainz K, Ulibarri H, Arroyo A, et al. Meta-analysis of maternal and neonatal outcomes of cannabis use in pregnancy current to March 2024. Matern Health Neonatol Perinatol. 2025;11(1):20. doi:10.1186/s40748-025-00216-9 ↩
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  7. Kumar AR, Sheikh ED, Monson JW, et al. Understanding the mechanism and extent of transplacental transfer of (−)-Δ9-tetrahydrocannabinol (THC) in the perfused human placenta to predict in vivo fetal THC exposure. Clin Pharmacol Ther. 2023;114(2):446–458. doi:10.1002/cpt.2964 ↩
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