Science Update: Cannabis use in pregnancy may impair placental function, suggests NIH-funded study

Cannabis associated with hypertensive disorders and other conditions linked to placental health

Someone holding a marijuana leave in four fingers of their upturned hand.
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Compared to pregnant people who tested negative for cannabis use during pregnancy, pregnant people who tested positive had a 27% higher risk for disorders associated with poor functioning of the placenta, including hypertensive disorders of pregnancy or having an infant born small for gestational age, suggests a study funded by the National Institutes of Health.

To increase the chances of a healthy pregnancy and newborn, the authors cautioned against cannabis use during pregnancy.

The study was conducted by Torri Metz, M.D., of the University of Utah Health, and colleagues. It appears in the Journal of the American Medical Association. Funding was provided by NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development.


A previous report found that cannabis use during pregnancy is linked to lower offspring birthweight. A study of rhesus macaques indicated that animals exposed to cannabis had placentas with lower blood flow and less oxygen than placentas of unexposed animals.

Earlier studies suggest that cannabis use among pregnant people may be underreported. Questionnaire responses on cannabis use in pregnancy typically underestimate actual use by 2- to 3-fold when compared with hospital screens of urine and tissue samples at hospital admission. This underreporting, the authors stated, make it difficult to understand the potential effects of cannabis during pregnancy.

For the current study, the researchers compared levels of a cannabis by-product in urine samples that participants provided in an earlier study. Samples were collected at three intervals: from 6 through 13 weeks (the first trimester), 16 weeks through 21 weeks, and 22 weeks through 29 weeks. Researchers compared a positive test result to the odds of complications thought to be related to impaired placental functioning: small-for-gestational-age birth, medically indicated preterm birth, stillbirth, or hypertensive disorders of pregnancy.


Of the 9,257 participants in the analysis, 610 (6.6%) tested positive for cannabis, 197 (32.4%) were positive only during the first trimester, and 413 (67.6%) tested positive beyond the first trimester.

Of those who tested positive at any time in the study, 25.9% had one or more of the complications linked to placental impairment, compared to 17.4% of those who tested negative, a higher risk of 27%. Cannabis use during the first trimester only was associated with an 18% higher risk for a placenta-linked condition. Cannabis use throughout pregnancy was associated with a 32% higher risk.


The authors said their findings suggest cannabis use should be avoided during pregnancy to optimize maternal and newborn outcomes.


Metz, T.D., et al. Cannabis exposure and adverse pregnancy outcomes related to placental function. Journal of the American Medical Association. 2023.

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