The prime suspect in the stabbing of Abigail Witchalls would have faced criminal charges if he had not killed himself shortly after the attack.
Richard Cazaly had been hunting in woods on the day of the attack
The Crown Prosecution Service (CPS) said there was "sufficient evidence" to prosecute Richard Cazaly, 23.
Mrs Witchalls was stabbed in the neck in front of her young son in April.
Det Supt Adrian Harper said: "The investigation is concluded. All the evidence points to Richard Cazaly as the offender."
The Surrey Police chief said the decision would be "difficult" for friends and family of Cazaly to accept and added: "There is no prior evidence to suggest he was capable of an act such as this."
He said Mr Cazaly became psychotic and violent as a result of long-term abuse of drugs and the alcohol he had consumed on the day of the stabbing.
Mrs Witchalls gave birth naturally to her second son, Dominic
Mrs Witchalls' father welcomed the announcement and said the family extended their sympathy and prayers to the Cazaly family.
He said police had been thorough and sensitive in dealing with the case and asked for continued respect for the family's privacy.
Mrs Witchalls, 26, was left partially paralysed after the attack in Little Bookham, Surrey, but has since given birth to her second child.
Mr Cazaly, of Fleet, Hampshire, died of a suspected overdose of painkillers on 30 April, having driven to Scotland five days after the stabbing.
He had been living in a house in Water Lane, Little Bookham, while working at a nearby garden centre.
No 'guilty declaration'
CPS principal legal adviser Chris Newell said: "The CPS has advised Surrey Police that their investigation into this offence revealed evidence that would have been sufficient to prosecute Richard Cazaly, if he were alive.
"But this is in no sense a declaration that he was guilty of the offence.
"Had Mr Cazaly lived, our decision would merely have authorised the police to begin the legal process by charging him.
"We agreed to look at the evidence on a deceased suspect because of the very exceptional circumstances of this case."
Mr Cazaly bore similarities to a description Mrs Witchalls gave of her attacker but she was unable to pick him out in a photo ID parade.
However, Surrey Police believed they had evidence which linked Mr Cazaly to the stabbing.
In May, four specialist dogs, one trained to sniff out human blood, searched the route Mrs Witchalls had been walking before she was attacked.
Police did not rule out the possibility that Mr Cazaly's scent was found.
They said in July that the evidence submitted to the Crown Prosecution Service would include information on the "process of the dogs going into the woods".
The file also included elements which could count against Mr Cazaly being the attacker, such as the fact that he was not identified in the ID parade.
Cazaly had lived an itinerant life since his late teens and was a heavy user of cannabis.
He normally worked on the other side of a hedge from the path Mrs Witchalls used on the afternoon of the stabbing to walk to a mother and toddler group.
At the time of the attack, Cazaly was breaking up with his Australian girlfriend Vanessa McKenzie, who looked similar to Mrs Witchalls.
Police spoke to Cazaly twice in the days after the attack - during local house-to-house inquiries and at a random road check.
His name was also given to officers by a suspicious neighbour but in the early stages of the investigation he was one of about 40 people police were looking at.