Ms Mills earlier claimed that her ex-partner, Darren Byrne, deliberately sat on her back when she was 21 weeks pregnant, causing the membrane of the amniotic sac protecting the foetus to rupture prematurely.
Mr Byrne strenuously denied the allegation at Dublin Coroner's Court.
But speaking after the inquest returned an open verdict, Gillian Mills said: "My son died for nothing. He died in agony for nothing."
The pair were giving evidence yesterday at the inquest into the death of Darren.
The post-mortem found he died as a result of hypoxic brain injury caused by a lack of oxygen passing through the placenta, and pneumonia due to an infection which came about as a result of the rupture.
Ms Mills alleged that three days before she presented at the Coombe on October 30, 2008, Mr Byrne assaulted her when she asked him to stop taking heroin.
He pushed her against the wall, she said, before pressing her face against the sofa. He then sat on her back "like I was an animal" until she fell to her knees. Ms Mills went to hospital three days later when she realised her waters were leaking.
There was a heated exchange in court, with Mr Byrne storming out of the inquest while shouting: "F*** her!" He returned, to deny under oath that any assault had taken place.
"Lies. There is no truth in it. I never sat on her," he said.
Consultant obstetrician Dr Hugh O'Connor said that pregnancies can continue for a number of weeks after rupture, but the baby is left open to infection.
Ms Mills gave birth on December 4, but the baby lived for only five-and-a-half hours.
Former master at Holles Street, Dr Peter Boylan, told the court that "on the balance of probabilities" the rupture could have been "precipitated or caused" by the assault as described by Ms Mills.
However, he said that while uncommon, it is possible for the membrane to rupture spontaneously at 21 weeks. Ms Mills had already had two normal pregnancies which made this less likely.
Garda Deirdre McMenamin said Ms Mills told her about the alleged assault in a private conversation the day after the baby's death, but she refused to make a complaint.
Coroner Dr Brian Farrell pointed to the "serious conflict" given by Ms Mills and Mr Byrne, saying that it could not be resolved by the inquest.