Stephen McCullagh had initially relied on a fake alibi that he was live-streaming a gaming session on YouTube on the night that he murdered Natalie McNally.
His alibi collapsed when the PSNI cyber crime unit discovered the six-hour video had been recorded days before the killing on 18 December 2022.
McCullagh was arrested and convicted of her murder after a five-week trial earlier this year.
He has been given a life sentence, with a minimum term of 31 years being set before he can be considered for parole.
McCullagh and Ms McNally were matched on the dating app Bumble in 2022. Two months after they met they were expecting a baby together.
Text messages shown during the trial revealed their plans to move in together and to spend Christmas 2022 with her family.
However, she was having second thoughts about the relationship and was still in contact with her former boyfriend, who McCullagh would later accuse of the murder.
There were also text messages between Ms McNally and other men, some revealing her hesitation about her relationship with McCullagh.
The prosecution made the case that, as he had the passcode to her phone, he could have seen these messages and been left "deceived, hurt, angry and enraged".
That rage was channelled into a carefully constructed murder plot.
Watch: Stephen McCullagh faked a six-hour live stream on YouTube
Four days before the murder, McCullagh pre-recorded a six-hour video of himself playing games, calling it "The Violent Night Christmas Live Gaming Stream".
It showed him playing Grand Theft Auto Vice City and Robot Wars.
On the night of the murder, he published it on his YouTube channel, billing it as live.
While the footage played, McCullagh tried to disguise himself and, as CCTV showed, travelled by bus to Lurgan.
He walked to Ms McNally's home, where he murdered her in a prolonged and vicious assault.
She was beaten, strangled and stabbed. Her body was found lying face down in a dog bowl.
McCullagh then got a taxi home. It is believed that he had missed the last train to Lisburn.
Watch: CCTV shows Stephen McCullagh travelling to and from Natalie McNally's home
He returned to her house the following evening to find her body and call 999.
In the call to the emergency services, McCullagh can be heard crying, saying: "There’s blood everywhere" and "her body is cold".
PSNI Detective Chief Inspector Neil McGuinness said: "To have been found in the circumstances she was found in was absolutely chilling, even for an experienced police officer."
He described her murder as "particularly brutal" and "particularly shocking for many, many reasons".
"You know, this was a young lady in what was supposed to be the safety of her own home.
"She was 15 weeks' pregnant with her unborn son, and she had just spent the evening with her family watching the World Cup football match," he said.
In the days and weeks following Ms McNally's murder, he spent time with her family and secretly recorded their conversations in his absence.
The family treated him as a grieving boyfriend and father-to-be, and gave him time alone with Ms McNally’s remains at her wake, spending 15 minutes in the room.
Read more:
Natalie McNally's partner sentenced to life for murder
McNally murder is 'life sentence' for family, father says
As it happened: McCullagh must serve 31 years in prison
Listen back to the 'Behind the Story' podcast:
How murderer Stephen McCullagh's alibi unravelled
The Public Prosecution Service said that McCullagh had "ingratiated himself with the family, attending a rally about violence against women and girls, preparing the compilation video for that".
"Really, his callousness knows no bounds, it seems," said head of the PPS Serious Crime Unit Catherine Kierans.
She said the murder was "unprecedented, in terms of the level of premeditation and planning, and the callousness of it".
McCullagh’s computer was seized by the PSNI. Its cyber crime unit forensically examined it and found the gaming session was not live-streamed.
Ms Kierans said that "proving the difference between a video recorded live and a video that's pre-recorded is quite difficult in terms of that video being on YouTube, and it really required the best and the brightest at the PSNI cyber crime unit to discern the difference between those".
When the expert evidence was presented to McCullagh, he admitted "in his final interview that the live stream was not live, it was fake, and at that point his entire case crumbled, really," she said.
While admitting the footage had been pre-recorded, he denied murder, claiming that he had been drunk and asleep on a sofa in his home as the video played out.
But the jury did not believe him and he was convicted by an unanimous verdict of the murder of Ms McNally in March.
