Children's Hospital Colorado
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Marijuana Breast Milk Research


A close-up of a baby bottle full of breast milk on a table with the mom holding baby in the background.

Fifty years ago, there wasn't much data on the risks of using alcohol or tobacco while pregnant or nursing — and because both are legal, many people assumed it was more or less safe. These days, the same is becoming true for marijuana.

"It's definitely been an issue over the last five years, especially since legalization in Colorado," says Maya Bunik, MD, MPH, Medical Director of the Primary Care and Breastfeeding Management Clinic at Children's Hospital Colorado. "There's a public perception that legalization means safety."

Marijuana safety myth or truth?

In reality, the meager body of existing scientific research has produced inconclusive and conflicting results. The problem, says Children's Colorado neonatologist Erica Wymore, MD, MPH, is that many people want it to be safe.

Pregnant and nursing women, for example, often say that marijuana helps with nausea and balancing the mood. And they tend to use it — sometimes in lieu of medications with existing safety profiles for pregnancy and breastfeeding — because they believe it's "natural." Research has indicated that marijuana is the most commonly used drug during pregnancy and nursing. Some estimates place use rates as high as 30%.

That's potentially a problem, says Dr. Wymore, because marijuana is known to have at least some toxicity, although what that might mean for pregnant and nursing women — and their babies — remains to be investigated.

"There's a real tendency to interpret the lack of data as an endorsement," she says. "But to say there aren't any consequences would be inaccurate. Up until now, there just hasn't been much previous research documenting safety. We haven't done robust, high-quality studies yet."

Marijuana effects on breastfeeding

Dr. Wymore is doing at least one of them. Along with Dr. Bunik, she leads a study at Children's Colorado, University of Colorado Hospital, and the University of Colorado School of Medicine investigating how much marijuana makes its way into breast milk and how long it stays there. Commissioned by the Centers for Disease Control, it's an early foray into an area of intense public inquiry and few definitive answers.

Because delta-9 tetrahydrocannabinol (THC), the psychoactive component in marijuana, attaches to fat, scientists have long presumed it would be present in the breast milk of mothers who use marijuana. Many doctors currently advise nursing mothers who have used the drug to "pump and dump" for two weeks.

"But that's just a guesstimate," says Dr. Bunik.

Current study hopes to shed light

The study is currently recruiting mothers who intend to breastfeed and who have a positive urine sample for isolated marijuana use or who volunteer a positive urine sample within five days of labor and delivery at Children's Colorado or University of Colorado Hospital. From there, the mothers complete a survey on the quantity they typically use, as well as frequency and mode of ingestion — smoked or edible — and agree to submit to breast milk, urine and blood tests two to five times per week over a period of six weeks. They also agree to abstain from marijuana over that period.

Like so much involving marijuana these days, the logistics of the study are complicated. The approval process alone took nearly a year, and protocols substantially changed. Initial plans included testing babies' urine in the study; however, in Colorado, a positive drug test in a child mandates a report to child protective services (for perspective, Washington state, where recreational marijuana is also legal, has no such requirement). Wanting participants to be honest without fear of legal ramifications, Drs. Wymore and Bunik scrapped that component.

Still, while the study protects its participants, recruitment is a challenge. Many mothers may be reluctant to submit to testing or admit to using marijuana while pregnant. Complicating matters further still is that, of the two mothers so far enrolled in the study, both have been unable to abstain for the six-week period.

"It speaks to how difficult this type of study is," says Dr. Bunik. "And it's concerning that moms are having difficulty abstaining during breastfeeding and pregnancy."

Over time, Drs. Wymore and Bunik hope to recruit 20 new mothers with the ultimate goal accurately mapping how THC presents in breast milk, and in what concentrations, and for how long.

It's a small step toward a more complete understanding of the health consequences of marijuana that Drs. Wymore and Bunik hope one day will help women make informed decisions about when, how, and if they use. That future body of information stands to exert a huge influence on consumer behavior in the long term — just as it did with alcohol and tobacco.

"Just because they [babies] don't have a major birth defect or overt withdrawal symptoms doesn't mean the baby's neurological development is not impacted."
- Erica Wymore, MD

Dr. Wymore is featured in a New York Times article on pregnant women who have chosen to use marijuana for various reasons. The article explains how these moms think it is safe to use in small amounts; however, they may actually be harming their infants by doing so.

"We want to support our patients in these decisions," says Dr. Wymore. "In the meantime, it's a good idea to be cautious."

Additional Resources

Colorado Department of Public Health and Environment

American Academy of Pediatrics: Breastfeeding and the Use of Human Milk, 2012 and Prenatal Substance Abuse: Short- and Long-term Effects on the Exposed Fetus, 2013

Academy of Breastfeeding Medicine (.pdf): Guidelines for Breastfeeding and Substance Use or Substance Use Disorder, ABM Clinical Protocol #21, 2015