Can cannabis use during pregnancy contribute to low birth weight?

Medically reviewed by Lauren Vrabel, PharmD

Illustrative photo by Christian Bowen on Unsplash

Cannabis is one of the most commonly used substances during pregnancy. Stigmas around weed are subsiding and the plant is becoming more prominent, leading to a rise in cannabis use. Pregnant women are no exception.

An estimated 4.2% of women use cannabis during pregnancy, although research suggests the number could be as high as 12.1% in certain states such as Maine. That number is also increasing. According to one study, cannabis use among pregnant women increased from 3.4% in 2002 to 7% in 2017. Women turn to weed during pregnancy to ease a number of common and often unavoidable symptoms such as stress, anxiety, nausea, and pain. ¹ ²

However, while it’s evident that a growing proportion of pregnant women use cannabis, what’s less clear is how cannabis can affect the fetus. One aspect of fetal health that has recently attracted attention is whether cannabis exposure in utero can impact birth weight.

Pregnancy, the endocannabinoid system, and cannabis

The endocannabinoid system (ECS) is integral to creating a healthy environment for the fetus during early pregnancy, playing a significant role in the development of the embryo, its implantation in the womb, the development of the placenta, and the formation of the brain and organ tissue. ³

There’s also evidence that in addition to its role in early pregnancy, the ECS contributes to the maintenance of the pregnancy and the timing of labor. The body’s endocannabinoid levels fluctuate in response to different phases of pregnancy. Anandamide levels, for example, decrease in the second and third trimesters but significantly surge during labor.

Researchers have found that exogenous cannabinoids, such as THC and CBD, can cross the placenta. There’s also evidence that these phytocannabinoids can impact the endocannabinoid signaling in the placenta and its development. For example, one study found that THC altered placental anandamide levels. What’s less clear, however, is whether these alterations can lead to harm or permanent damage. ⁴ ⁵

Potential effects of THC on pregnancy and birth weight

According to the World Health Organization’s growth standards, the average birth weight is 7 pounds, 6 ounces (3.3kg) for a full-term male baby and 7 pounds 2 ounces (3.2kg) for a full-term female baby.

However, babies come in all shapes and sizes, and the acceptable birth weight range is fairly broad. Most babies born between 37 and 40 weeks weigh between 5 pounds, 8 ounces (2.5kg), and 8 pounds, 13 ounces (4kg). Low birth weight is a term used to describe babies born weighing less than 5 pounds 8 ounces (2.5kg).

Research findings on cannabis and birthweight

An international study published in 2020 followed 5610 pregnant women with low risk pregnancies and categorized them based on their cannabis use patterns. Sixty women (a total of 1.9%) reported using weed at the 15-week mark. The study reported that women who had never used cannabis gave birth to babies who weighed a mean weight of 3.41kg, compared with women who were using cannabis at 15 weeks and gave birth to babies with a mean weight of 2.93kg. ⁶

Women who used cannabis no more than once a week gave birth to babies with an average weight of 3.39kg, while those who used weed more than once a week gave birth to babies with a mean weight of 3.06kg.

However, the study also revealed other findings. Women who were using cannabis at 15 weeks also gave birth to babies with smaller head circumferences (an average of 0.5cm smaller), who were shorter (a mean reduction of 0.8cm) and had a reduced gestational age (a mean of 278 days for mothers who had never used cannabis, compared to 270 days for mothers who were still using cannabis at 15 weeks). And while smaller babies might sound a little easier to birth naturally, this reduced birth weight and size is considered by experts to be a negative. The authors concluded that continued cannabis use during pregnancy represented a risk factor for poorer neonatal outcomes.

A 2022 systematic review and meta-analysis also offered insights into whether cannabis use during pregnancy leads to lower birth weight, preterm birth, or smallness for gestational age. The meta-analysis reviewed 32 studies containing data from approximately 5.5 million women. The study’s authors found that cannabis use led to an increased risk of low birth weight, preterm birth and smallness for gestational age, independent of the effects of other drugs or tobacco. ⁷

The findings also reported that the few studies that assessed the timing and frequency of cannabis consumption suggested a dose-response effect. Women who disclosed heavy use and continued use during the second and third trimester of gestation experienced higher odds of negative outcomes.

While these studies suggest that women who use cannabis (particularly those who use it regularly) are more likely to give birth to babies with lower birth weights, the mean birth weights documented in the 2020 study still fall within what is considered to be a normal weight range (2.5-4kg).

Similarly, gestational age at birth also falls into an acceptable range spanning 38 to 42 weeks. By this measure, the mothers in the study who were using cannabis at 15 weeks and had a mean gestational age of 38.5 weeks still largely fell within this safe range.

Can cannabis use lead to neurodevelopmental disorders in offspring?

Besides lower birth weight, cannabis use during pregnancy has also been loosely linked to higher rates of neurodevelopmental disorders such as ADHD, anxiety, and depression. For example, a 2021 study of 322 mothers and children found that cannabis exposure in utero corresponded to widespread reductions in immune-related gene expression. These reductions were believed to increase cortisol, anxiety, aggression, and hyperactivity in children. ⁸

However, the findings on cannabis exposure in utero and childhood neurodevelopmental disorders are far from settled and are characterized by conflicting results. One recent Canadian cohort study found that cannabis use during pregnancy does not lead to an increased risk of ADHD in children. ⁹

Other research has found that children with neurodevelopmental and neuropsychiatric disorders may even benefit from treatment with exogenous cannabinoids such as CBD. Most importantly, much of the research exploring the effects of cannabis exposure in utero acknowledges that the findings are affected by significant limitations—which will be explored more below. ¹⁰

Putting existing evidence in perspective: The limitations of existing data

Studies evaluating the effects of cannabis use in pregnancy tend to report mixed results for several reasons. An awareness of the limitations of the existing evidence can help to contextualize it and explain how it might not convey the full picture. ¹¹

Weak methodology

Much research on cannabis exposure in utero relies on self-reported use and surveys. However, this research methodology can be compromised by participants giving more socially acceptable answers rather than truthful responses. Self-reported research can also be limited by participants’ ability to assess themselves accurately, including both personal and cultural biases. ¹²

Research also often relies on urine tests to identify women who use cannabis while pregnant. This test shows the presence of THC but doesn’t assess when or how much THC was consumed. As the 2022 systematic review and meta-analysis showed, there’s a dose-response effect between cannabis use in pregnant women and adverse outcomes that needs to be taken into consideration. ¹³

What’s more, many existing studies don’t identify the delivery method or type of cannabis pregnant women use.Cannabis can be broadly categorized into three main types: Type I (THC dominant), Type II (balanced THC and CBD), and Type III (CBD dominant). Nearly all studies exploring the effects of cannabis exposure in utero refer generically to cannabis but don’t differentiate or investigate the effects of specific cannabinoids. While THC may affect the developing placenta, it’s less clear how CBD affects a fetus, for example.

In recent years, other forms of cannabis consumption have gained popularity, such as edibles and tinctures. While the active ingredients remain the same and the developing fetus may still be exposed, the potency and pharmacokinetic profiles of different delivery methods vary and may affect the fetus differently. ¹⁴

Confounding factors

Research exploring the effects of substances on human participants can be challenging due to confounding factors. Confounding factors mean unmeasured third variables that can influence the supposed cause and effect.

Studies on cannabis use during pregnancy often emphasize that it’s challenging to rule out other variables as diverse as alcohol use, body mass index (BMI), illicit drug use, age, symptoms of anxiety and depression, and socio-economic background. In other words, it’s unclear the degree to which cannabis contributes to low birth weight in a child whose mother is also underweight, impoverished, and consumes alcohol regularly.

Summary

Overall, it’s clear that we still have much to learn about the effects of cannabis on the developing fetus and birth weight. As such, researchers and healthcare providers usually encourage pregnant women to be conservative with cannabis use or abstain completely, since there is no known safe level of use during pregnancy at present. ¹⁵

However, it’s also vital to critically and thoughtfully examine existing research as it relates to potential risk. While the existing data suggests that cannabis use can contribute to low birth weight, the mean difference doesn’t fall into a range that’s dangerous or remarkably abnormal. What’s more, there appears to be a dose-response relationship that should also be taken into account. More regular cannabis use may increase the likelihood of a low birth weight, but existing evidence suggests less risk with less use. Until we have more reliable, robust data, it’s difficult to draw firm conclusions, but it is reasonable to say that mothers should use caution when using cannabis, and if it is necessary, use the lowest possible dose needed to achieve the desired effect.

Sources

Navarrete, F., García-Gutiérrez, M. S., Gasparyan, A., Austrich-Olivares, A., Femenía, T., & Manzanares, J. (2020). Cannabis Use in Pregnant and Breastfeeding Women: Behavioral and Neurobiological Consequences. Frontiers in psychiatry, 11, 586447. https://doi.org/10.3389/fpsyt.2020.586447
Young-Wolff, K. C., Ray, G. T., Alexeeff, S. E., Adams, S. R., Does, M. B., Ansley, D., & Avalos, L. A. (2021). Rates of Prenatal Cannabis Use Among Pregnant Women Before and During the COVID-19 Pandemic. JAMA, 326(17), 1745–1747. https://doi.org/10.1001/jama.2021.16328
Kozakiewicz, M. L., Grotegut, C. A., & Howlett, A. C. (2021). Endocannabinoid System in Pregnancy Maintenance and Labor: A Mini-Review. Frontiers in endocrinology, 12, 699951. https://doi.org/10.3389/fendo.2021.699951
Martínez-Peña, A. A., Perono, G. A., Gritis, S. A., Sharma, R., Selvakumar, S., Walker, O. S., Gurm, H., Holloway, A. C., & Raha, S. (2021). The Impact of Early Life Exposure to Cannabis: The Role of the Endocannabinoid System. International journal of molecular sciences, 22(16), 8576. https://doi.org/10.3390/ijms22168576
Maia, J., Midão, L., Cunha, S. C., Almada, M., Fonseca, B. M., Braga, J., Gonçalves, D., Teixeira, N., & Correia-da-Silva, G. (2019). Effects of cannabis tetrahydrocannabinol on endocannabinoid homeostasis in human placenta. Archives of toxicology, 93(3), 649–658. https://doi.org/10.1007/s00204-019-02389-7
Grzeskowiak, L. E., Grieger, J. A., Andraweera, P., Knight, E. J., Leemaqz, S., Poston, L., McCowan, L., Kenny, L., Myers, J., Walker, J. J., Dekker, G. A., & Roberts, C. T. (2020). The Medical journal of Australia, 212(11), 519–524. https://doi.org/10.5694/mja2.50624
Baía, I., & Domingues, R. (2022). The effects of cannabis use during pregnancy on low birth weight and preterm birth: a systematic review and meta-analysis. American journal of perinatology, 10.1055/a-1911-3326. Advance online publication. https://doi.org/10.1055/a-1911-3326
Rompala, G., Nomura, Y., & Hurd, Y. L. (2021). Maternal cannabis use is associated with suppression of immune gene networks in placenta and increased anxiety phenotypes in offspring. Proceedings of the National Academy of Sciences of the United States of America, 118(47), e2106115118. https://doi.org/10.1073/pnas.2106115118
Tchuente, V., Sheehy, O., Zhao, J. P., Gorgui, J., Gomez, Y. H., & Berard, A. (2022). Is in-utero exposure to cannabis associated with the risk of attention deficit with or without hyperactivity disorder? A cohort study within the Quebec Pregnancy Cohort. BMJ open, 12(8), e052220. https://doi.org/10.1136/bmjopen-2021-052220
Kwan Cheung, K. A., Mitchell, M. D., & Heussler, H. S. (2021). Cannabidiol and Neurodevelopmental Disorders in Children. Frontiers in psychiatry, 12, 643442. https://doi.org/10.3389/fpsyt.2021.643442
Thompson, R., DeJong, K., & Lo, J. (2019). Marijuana Use in Pregnancy: A Review. Obstetrical & gynecological survey, 74(7), 415–428. https://doi.org/10.1097/OGX.0000000000000685
Thompson, R., DeJong, K., & Lo, J. (2019). Marijuana Use in Pregnancy: A Review. Obstetrical & gynecological survey, 74(7), 415–428. https://doi.org/10.1097/OGX.0000000000000685
Baía I, Domingues RMSM. The Effects of Cannabis Use during Pregnancy on Low Birth Weight and Preterm Birth: A Systematic Review and Meta-analysis [published online ahead of print, 2022 Sep 16]. Am J Perinatol. 2022;10.1055/a-1911-3326. doi:10.1055/a-1911-3326
Martínez-Peña, A. A., Perono, G. A., Gritis, S. A., Sharma, R., Selvakumar, S., Walker, O. S., Gurm, H., Holloway, A. C., & Raha, S. (2021). The Impact of Early Life Exposure to Cannabis: The Role of the Endocannabinoid System. International journal of molecular sciences, 22(16), 8576. https://doi.org/10.3390/ijms2216857
Badowski, S., & Smith, G. (2020). Cannabis use during pregnancy and postpartum. Canadian family physician Medecin de famille canadien, 66(2), 98–103.