1. Duration of Marijuana in the Body. When mothers smoke marijuana and breastfeed, the active ingredient in marijuana –tetrahydrocannabinol or THC – stays in the mother’s body for four to six weeks and it is passed on and found in the urine of breastfeeding babies.urthermore, marijuana use can diminish the abilities of a mother to take care of her newborn. The American Academy of Pediatrics advises nursing mothers not to use marijuana while breastfeeding. However, in some states medical marijuana is used to treat anxiety and other medical conditions.