Myrtle Beach police charged Marissa Ann Joseph, 22, of Myrtle Beach with unlawful neglect of a child.
The Department of Social Services was investigating Joseph and the baby's father, Jonathon Hoard, 26, for allegations of using drugs.
Drug test results showed both parents tested positive for marijuana. The baby tested positive for marijuana and cocaine.
The police report says its unclear how the baby consumed cocaine when the parents denied having or using cocaine around the child.
A grandmother took custody of the baby.
Myrtle Beach police arrested Hoard Monday.