Police errors had a detrimental impact on the investigation into the disappearance of murdered teenager Jayden Parkinson, a report says.
The 17-year-old's remains were found in a disturbed grave in All Saints' Church, Didcot, in December 2013.
Failures to record information before her death meant she was not treated as a high priority when she went missing, the IPCC police watchdog said.
Thames Valley Police said it would act on lessons learned.
A report by the Independent Police Complaints Commission (IPCC) said while the force "could not have prevented her tragic death" it also failed to allocate enough staff to follow up earlier enquiries about Jayden.
It was nearly a week before Jayden was identified as a high risk.
IPCC associate commissioner Guido Liguori said: "Jayden was clearly vulnerable when she came to police attention a number of times prior to her death.
"If information about these interactions had been correctly recorded on police systems, Jayden would have been treated as a high priority when she later went missing. "
The force has been urged by the IPCC to improve the recording of information on force computer systems.
Ben Blakeley, 23, from Reading, the ex-boyfriend of the pregnant 17-year-old, was jailed for life in July last year for her murder.
Three officers were disciplined over an inquiry into Jayden's disappearance in October.
Two officers received a final written warning, the third a written warning. A civilian staff member was found to have "no case to answer" over the same investigation.
Thames Valley Assistant Chief Constable Chris Shead said: "We are determined... to act upon any lessons to be learnt.
"Jayden's family continue to go through an incredibly difficult time, and I again offer my condolences to them."
The force said the Oxfordshire Safeguarding Children's Board had commissioned a serious case review, combined with a domestic homicide review due to the circumstances of the case.
Jayden's mother Samantha Shrewsbury criticised the police for not issuing an apology following the report.
"I'm angry because I feel if they had done their job and listened to what we were saying about their relationship, that she would still be here now.
"We weren't taken that seriously. I'm glad that they've learnt from that lesson but it still doesn't make a difference for my Jayden."