The research: Fifty nursing moms who regularly use marijuana donated breastmilk samples to the Mommy’s Milk Human Milk Research Biorepository, at the University of California, San Diego. Traces of THC (tetrahydrocannabinol), the primary psychoactive component of marijuana, were found in 63 percent of the samples up to six full days after the mom’s last reported use, according to a study in Pediatrics.