When the baby's father, Dwayne Bryant, tried to wake his girlfriend, Yadina Morales, he found the lifeless body of his daughter trapped under her.
DailyMail reports that the 22-year-old woman, who has been charged with manslaughter, was apparently so drunk she didn't even realize her child had been rushed to hospital.
'We found that the mother was highly intoxicated and had actually been breastfeeding the child and passed out and ultimately smothered the child,' Sheriff Douglas Mullendore told NBC Washington.
The baby was rushed to hospital but it was too late to save her. A post-mortem examination is now due to be carried out in Baltimore.
According to court records, the baby's father, Mr Bryant, called emergency services who found him screaming in the car park of the couple's home, Business Insider reported.
'She had no idea what was going on. She didn't even know EMS had been there and took her child to the hospital,' a police deputy said in court documents.
A blood test found Morales, who has two other children, was three times over the legal drink-drive limit.