Medical Cannabis and Pregnancy: What UK Women Need to Know

Introduction

Medical cannabis is contraindicated during pregnancy. This is one of the clearest and most consistent positions in UK and international clinical guidance, and it is important that any woman who is pregnant, planning a pregnancy, or breastfeeding understands this before considering or continuing a cannabis-based medicinal product (CBPM) prescription.

This guide explains why medical cannabis is not recommended during pregnancy, what the evidence says about risks to the developing baby, and what UK women should do if they are currently prescribed CBMPs and become pregnant.

Why Medical Cannabis Is Contraindicated in Pregnancy

The primary concern relates to THC (tetrahydrocannabinol), the psychoactive cannabinoid present in most prescribed cannabis products. THC crosses the placental barrier and can reach the developing foetus. It also passes into breast milk.

The endocannabinoid system (ECS) plays a critical role in foetal brain development, including neuronal migration, synapse formation, and the development of the prefrontal cortex. Disruption of the ECS during this sensitive period — even by exogenous cannabinoids — carries potential risks.

What Does the Evidence Say?

The evidence on cannabis use in pregnancy comes primarily from studies of recreational cannabis use, as it would be unethical to conduct controlled trials involving pregnant women. The key findings include:

Foetal growth and birth weight: Multiple large cohort studies have found associations between cannabis use during pregnancy and reduced foetal growth, lower birth weight, and preterm birth. A 2020 meta-analysis of 59 studies published in JAMA Pediatrics found that cannabis use during pregnancy was associated with a 50% increased risk of low birth weight.

Neurodevelopmental outcomes: Children born to mothers who used cannabis during pregnancy show higher rates of attention problems, impulsivity, anxiety, and lower academic achievement in later childhood. These associations persist after controlling for confounding factors such as tobacco use and socioeconomic status.

Stillbirth: A 2019 study published in Obstetrics & Gynecology found that cannabis use during pregnancy was associated with a significantly increased risk of stillbirth, independent of tobacco use.

CBD: The evidence on CBD specifically in pregnancy is much more limited, but animal studies have raised concerns about potential effects on foetal development even at low doses. The UK's Medicines and Healthcare products Regulatory Agency (MHRA) advises against CBD use during pregnancy and breastfeeding.

What Should You Do If You Are Pregnant and Prescribed Medical Cannabis?

If you are currently prescribed a CBPM and become pregnant — or are planning a pregnancy — you should:

1. Inform your prescribing clinician immediately. Your prescription will need to be reviewed and almost certainly discontinued or replaced with a safer alternative. 2. Do not stop abruptly without medical advice. If you have been prescribed cannabis for a serious condition such as epilepsy or severe chronic pain, abrupt discontinuation may carry its own risks. Your clinician will help you taper safely and identify alternative treatments. 3. Inform your midwife or obstetrician. They need to know about all medications and supplements you are taking, including CBMPs. 4. Do not use CBD products purchased over the counter. These are not regulated as medicines and their safety in pregnancy has not been established.

What About Breastfeeding?

THC is detectable in breast milk for up to six days after use, and potentially longer with regular use. CBD is also present in breast milk. The long-term effects of cannabinoid exposure through breast milk on infant neurodevelopment are not yet fully understood, but current guidance from the NHS, MHRA, and the Royal College of Obstetricians and Gynaecologists (RCOG) advises against cannabis use of any kind during breastfeeding.

How CPGUK Can Help

CPGUK provides free, confidential guidance to patients navigating the intersection of medical cannabis prescribing and pregnancy. If you are pregnant or planning a pregnancy and are currently prescribed a CBPM, please contact us. We can help you understand your options, communicate with your prescribing clinician, and access appropriate alternative care.

Conclusion

Medical cannabis — including CBD products — is not recommended during pregnancy or breastfeeding. The evidence, while largely observational, consistently points to risks of reduced foetal growth, neurodevelopmental effects, and adverse birth outcomes. Any woman who is pregnant or planning a pregnancy should discuss her CBPM prescription with her clinician immediately and seek safe alternatives for managing her condition during this period.