"Your daughter told you the baby was suffering from breathing problems," the judge said. "You had time to get first aid."
Fritzl said: "I was hoping the little one would survive but I should have done something. I don't know why I didn't help. I just lost sight [of the issue]."
He then said he was "sorry".
Speaking later outside the court, Mr Mayer said the testimony given by his client's daughter had allowed Fritzl to see for the first time the impact of his actions.
Describing his client, Mr Mayer said Fritzl was "a person who had only one idea - 'I must always be full of power'".
Mr Mayer said he was "very, very surprised" by Fritzl's plea reversal, but that Fritzl hoped his change of plea would help his victims.
The court later heard psychiatrist Dr Adelheid Kastner say there was a danger Fritzl would repeat his behaviour if he was left untreated.
"He will remain the same person, and the ways to exercise this control will change and change with his physical abilities, but his needs will remain the same," she told the BBC outside the court.
"So he will be a danger and he has to be kept in prison until he is no danger for others."
She recommended that he be sent to a special facility for deranged criminals, although strictly speaking he was not insane.
Lawyer on defending Fritzl
"What I told the court was that Mr Fritzl has never been mentally ill," she said, "and that he has always been sane in the legal sense of the word - that he was always able to discern between right and wrong, and that he always knew what he did was wrong."
She said Fritzl had an overwhelming need to dominate and control, which she said stemmed from his childhood.
She said he had been an unwanted, unloved child, who was intelligent, and who had grown up determined to have somebody who belonged to him alone.
He was emotionally deficient but he knew that what he was doing was wrong, she added.
Two technical experts then gave evidence, describing the cellar into which Fritzl lured his 18-year-old daughter beneath their house in Amstetten in 1984.
AFP news agency quoted the experts as saying they had found no mechanism in the cellar to open the doors and let the prisoners out if something happened to Fritzl - a feature that he had initially claimed existed.
He imprisoned his daughter in the cellar and raped her repeatedly over a number of years.
The daughter and three of the children fathered by Fritzl were kept captive in the cellar until the case came to light in April last year, when one of the children became seriously ill and was taken to hospital.
He was accused of murdering by neglect one of newborn twin boys his daughter gave birth to in 1996, having failed to arrange medical care for the ailing infant.
A former tenant of Fritzl has told the BBC he saw nothing amiss while he was renting a flat in his house from 1998-1999, although he suspected his landlord was taking electricity from the flat.
Franz, who still lives in Amstetten, said he remembered Fritzl as someone "really very strange" who would never let his wife speak whenever they met.
Fritzl's garden and basement were off limits to tenants, he said.
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