THC is detectable in breast milk for up to six weeks

“With the increasing utilization of marijuana in society as a whole, we
are seeing more mothers who use marijuana during pregnancy,” said
researcher Erica Wymore [assistant professor of pediatrics at CU
School of Medicine]. “However, given the lack of scientific data
regarding how long THC persists in breast milk, it was challenging to
provide mothers with a definitive answer regarding the safety of using
marijuana while breastfeeding and simply ‘pumping and dumping’ until
THC was no longer detectable in their milk.”