Gareeca Conita Gordon, 28, appeared before Bristol Crown Court via video link for a short hearing on Wednesday morning.
Gordon, who is in custody at Eastwood Park Prison in Gloucestershire, pleaded guilty to murdering Phoenix Netts.
Charges state that Gordon, of Salisbury Road, killed Ms Netts on a date between April 14 and May 12 last year.
Gordon, who spoke only to confirm her identity and plead guilty, had been due to stand trial at the court on Monday.
Police discovered Ms Netts' remains beside a quarry near Coleford, Gloucestershire, in the Forest of Dean, on May 12.
J+udge Peter Blair QC, the Recorder of Bristol, remanded Gordon into custody ahead of the sentencing hearing.
"In your case, Gareeca Gordon, you will be brought to court for your sentencing hearing on May 4," he said.
Following the death of Ms Netts, also from Birmingham, her family released a statement through West Midlands Police.
They said they were "devastated with what has happened" and asked for their privacy to be respected.
Gordon will be sentenced by High Court judge Mrs Justice Cutts at Bristol Crown Court on May 4.
In November, a Wolverhampton man accused of assisting an offender over the case had the charge against him dropped.
Mahesh Sorathiya was initially charged with the offence after Miss Netts' remains were found.