"It was an act of brutal, sustained and wholly unprovoked violence," Mr Jackson added.
"He knew he was not only killing her but their unborn child as well."
Residents living near the scene of the alleged attack in the town's Howard Way, minutes from Ms Blunnie's home, heard her "horrific" screams and the impact of the blows.
Ms Blunnie, who was due to give birth within days, was later found with severe head and facial injuries and died in hospital.
After the alleged killings, McLernon hid in nearby woodlands and sent a text message to his father saying "I'm scared".
He later spoke to his father on the phone and told him "I think I've killed Eystna", the court heard.