Christine Rivero, 29, of Collingdale, Delaware County, is charged with murder in the death of 11-month-old Niccolo Varner.
Toxicology reports revealed that the baby died because he ingested heroin back on July 15th.
According to police, Rivero initially told them she put the boy in the bath with his 5-year-old brother before he died.
Later, however, she allegedly admitted to leaving the children unattended to meet with friends to do heroin.
She returned to the house when the five-year-old started screaming that Niccolo had fallen in the tub.
Investigators initially thought the baby hit his head and drowned. But, since they found drug paraphernalia in the house, the coroner ordered toxicology tests.
On Tuesday, he ruled that the child had died of acute heroin intoxication.
Investigators say they are not sure how the child got ahold of the heroin, whether he found it in the house or was given the heroin.
"We cannot, at this point in time, explain how the child ingested that level of heroin into his system, or whether he was given heroin directly by the mother," said Delaware County D.A. Jack Whelan.
Rivero denies any involvement in Niccolo's death from a heroin overdose.
"I didn't do nothing to my child," she said as she left court on Friday. When asked if she had heroin in the house, she said "No, I didn't."
When asked how the baby had access to heroin, she said "I have no idea."
Back in July, a source told Action News that heroin paraphernalia was found in the home.
Rivero spoke to Action News back in July, shortly after Niccolo's death.
"He was the best. He was a good kid! He was the best baby in the world," said Rivero back on July 15th.
"When I went to pick him up, his chest was wet and his chest was blue. So I grabbed him out and started screaming for my husband," she said.
They attempted CPR, but Niccolo was gone.
"Babies don't just die," said Rivero at the time. "He was a very healthy baby. He was running around playing yesterday having fun."
Rivero is being held without bail on charges of third-degree murder, criminal homicide and involuntary manslaughter.
In addition, police say Rivero was arrested a few weeks ago in Sharon Hill on a DUI charge.