A review in Obstetrics & Gynecology concluded women should avoid marijuana during pregnancy and breastfeeding based on evidence of potential harm | Research

The takeaway

A comprehensive review found that marijuana crosses the placenta and passes into breast milk, and despite mixed evidence, animal and human data suggest potential harm including growth restriction, stillbirth, preterm birth, NICU admission, and long-term neurobehavioral effects in exposed children.

Obstetricians and midwives counseling pregnant patients; pregnant women considering cannabis use; pediatricians assessing exposed infants.

Marijuana crosses the placenta and passes into breast milk, with evidence of potential harm

What the researchers found

Researchers reviewed the literature on marijuana use during pregnancy and breastfeeding, published in a major obstetrics journal.

Key findings:

- Marijuana crosses the placenta and is present in breast milk, directly exposing the fetus and nursing infant.

- The endocannabinoid system plays important roles in implantation, placentation, and fetal neurologic development, making disruption biologically plausible.

- Two recent systematic reviews and meta-analyses found associations between marijuana use and adverse perinatal outcomes, especially with heavy use.

- Three longitudinal cohort studies demonstrated possible effects of prenatal exposure on long-term neurobehavioral outcomes in children.

- Marijuana use may be associated with growth restriction, stillbirth, spontaneous preterm birth, and NICU admission.

- Many women cited medical reasons for prenatal use: nausea/vomiting, anxiety, and chronic pain.

Limitations acknowledged: most studies are retrospective, rely on self-report (which underestimates use), and many fail to adequately control for tobacco and sociodemographic confounders.

Despite these limitations, the authors recommended women refrain from marijuana during pregnancy and lactation.

Why it matters

Cannabis use during pregnancy is increasing as legalization expands and perceived safety rises. This review from a leading obstetrics journal provides obstetricians with a comprehensive summary of the evidence and a clear recommendation to share with patients.

The numbers in context

Prevalence of prenatal marijuana use is increasing. Marijuana crosses the placenta and passes into breast milk. Two meta-analyses found adverse perinatal associations. Three longitudinal cohorts found potential neurobehavioral effects. Associations noted with growth restriction, stillbirth, preterm birth, and NICU admission.

How the study worked

Narrative review published in Obstetrics and Gynecology covering animal studies, human cohort studies, systematic reviews, and meta-analyses on marijuana in pregnancy and breastfeeding.

What this study cannot tell us

Narrative review. Underlying studies have significant methodological limitations including self-report bias, confounding by tobacco and socioeconomic factors, and lack of dose-response data. Definitive randomized trials are unethical in pregnancy.

How to read the evidence

Moderate. Published in a top OB/GYN journal with comprehensive coverage, but limited by the methodological weaknesses of underlying primary studies.

When this study was published

Published in 2018. Prenatal cannabis research has continued, with major medical organizations maintaining recommendations against use.

The bigger picture

The tension between insufficient evidence to definitively prove harm and enough evidence to warrant caution is a recurring challenge in prenatal substance research. The precautionary approach recommended here reflects the irreversibility of potential developmental effects.

Questions still open

How much marijuana exposure during pregnancy is harmful? Are there critical windows of vulnerability? Does CBD alone carry the same risks as THC? How should clinicians counsel pregnant women who use cannabis for severe nausea?

Common questions

Can marijuana help with pregnancy nausea?

Some women report using marijuana for nausea and vomiting of pregnancy. However, this review found potential risks to the fetus from marijuana exposure, and there are safer anti-nausea treatments available. Cannabinoid hyperemesis syndrome can also cause vomiting that mimics pregnancy-related nausea.

Does marijuana in breast milk affect the baby?

THC is fat-soluble and concentrates in breast milk. The effects on nursing infants are not well-studied, but given the developing infant brain's endocannabinoid system, the review authors recommended avoiding marijuana while breastfeeding.

Read the original research

Marijuana Use in Pregnancy and While Breastfeeding.

Obstetrics and gynecology, 132(5), 1198-1210

View on PubMed ↗Publisher’s paper ↗

Citation

Metz, Torri D; Borgelt, Laura M. (2018). Marijuana Use in Pregnancy and While Breastfeeding.. Obstetrics and gynecology, 132(5), 1198-1210. https://doi.org/10.1097/AOG.0000000000002878