By Lisa Rapaport

(Reuters Health) – Low levels of chemicals in marijuana like THC (tetrahydrocannabinol) can linger in breast milk for up to six days after nursing mothers use the drug, a U.S. study suggests.

Marijuana is the most commonly used illegal drug among pregnant and breastfeeding women. While some previous research suggests that prenatal marijuana exposure may impair fetal growth and brain development, less is known about the effects of marijuana on breastfeeding infants.

For the current study, researchers tested 54 milk samples from breastfeeding women who said they used marijuana. A form of THC was found in 34 samples up to about six days after the women last reported marijuana use, researchers report in Pediatrics.

“We have not had any data on what levels of cannabis metabolites are present in the milk of mothers who are recreational users and how long they might persist,” said senior study author Christina Chambers of the University of California San Diego and Rady Children’s Hospital in La Jolla, California.

“This study helps address that question,” Chamber said by email.

But it leaves a lot of other questions unanswered, including how harmful THC and other chemical byproducts of marijuana might be for breastfeeding babies.

There’s some urgency to find that answer because a growing number of women are using marijuana while pregnant and breastfeeding, Chambers said.

A study published last year in JAMA, for example, found the proportion of pregnant U.S. women using marijuana rose from about 4 percent in 2009 to 7 percent by 2016.

This surge was most pronounced for younger U.S. mothers. The proportion of pregnant women under 18 using marijuana climbed from 13 percent to almost 22 percent during the period examined in that study, while the proportion of pregnant women 18 to 24 years old using marijuana rose from 10 percent to 19 percent.

More women may be using cannabis while breastfeeding because they used it prior to pregnancy, said Dr. Sheryl Ryan of Penn State Health Milton S. Hershey Medical Center in Hershey, Pennsylvania.

“They may see no reason why they need to stop their use, in the absence of definitive data about dangers of its use during pregnancy,” Ryan, author of an accompanying editorial, said by email.

Marijuana use during pregnancy and breastfeeding may be rising in part because legalization of medical marijuana has made people think of the drug as less dangerous, Ryan said.

“There is general feeling in the population that if it is legal, then it must be safe, which is a fallacy,” Ryan added.

Doctors need to do a better job of making sure women understand the risks of marijuana during pregnancy, and the potential for it to reach babies through breast milk, according to recommendations from the American Academy of Pediatrics (AAP) published in conjunction with the study.

Women of reproductive age need to know that it’s unclear how THC exposure may affect them or their babies and be advised to avoid marijuana while breastfeeding, the AAP recommends. While women may get this guidance from their own doctors, they should also get it from pediatricians during well-baby checkups.

“What’s recommended is that women breastfeed, but abstain from using marijuana products while breastfeeding,” Ryan, also an author of the AAP recommendations, said. “We still do not know whether the amount of THC that an infant receives through breastmilk is safe, and in the absence of safety data, we recommend abstaining – this is what should be of importance for breastfeeding women.”

SOURCE:, and Pediatrics, online August 27, 2018.