Nicole McKibben yesterday told a Fatal Accident Inquiry that she heard Declan Hainey screaming for months on end.
His body was found in his litter-strewn cot at the home he shared with Kimberley in Paisley, Renfrewshire, in March 2010. She was found guilty of neglecting and murdering him and was jailed for life in 2012. But last year she was released on appeal after a judge ridiculed damning medical evidence from her trial.
The development meant Declan's death became unsolved, prompting the Crown Office to call for a Fatal Accident Inquiry to try and establish how he died. Yesterday, at the inquiry, Ms McKibben said: "I just thought he sounded distressed. I wish now I had done something about it.
"It's something I'll regret for the rest of my life."
The witness, who lived in the flat next to the one Declan shared with his mother, said that everything was normal when Ms Hainey and her son moved in at first at the end of 2008.
She said she would hear noises coming from their flat up until around the time of Declan's first birthday, in April 2009, but by summer and autumn, things become different.
She said: "There were things that started to change - the noises became more frequent and louder."
Ms McKibben said that Declan's crying changed.
"There was more and more crying - it started to make me question what was going on. All the crying, why there was so much noise.
"He was crying continually for months."
The 35-year-old, who got in from work about 6pm and would head to bed around 11pm, added: "All evening when I was in the house I could hear him."
The inquiry heard that, one day, all the noise just stopped.
She told procurator fiscal Stuart Cassidy: "I wish I had trusted my instincts more."
The inquiry into the death continues at Paisley Sheriff Court.