Shelby Anna Herchak, 18, has been charged with second-degree murder in the death of her infant son in Calgary.
Daniel Herchak was 26 days old when he died 10 hours after being rushed by paramedics to the Alberta Children's Hospital on Aug. 9.
The baby was killed by blunt-force trauma to the head, which an autopsy indicated was not accidental, police said.
Homicide detectives arrested Herchak on Tuesday afternoon and police announced the murder charge Wednesday.
"We believe the injuries were caused by one or more impacts with a solid object, or against a solid object," said Acting Insp. Doug Andrus.
A charge of second-degree murder involves the intent to commit murder, but not planning or deliberation, he added.
The baby's father, who is estranged from the mother, is not being investigated, police said.
"We have interviewed everyone who had access to the child. We don't believe anybody witnessed the incident," Andrus said.
There were several postings on the social networking website Facebook alleging Herchak had been at a nightclub in the hours leading up to her baby's death, Andrus said.
"It is known that she had been drinking in the past 24 hours. But whether that played a role in this incident, I can't speculate," he said.