Detectives said no cause of death has been determined for the 17-year-old, the Oxford Mail reported.
Detectives excavated a disturbed grave Wednesday in Great Western Cemetery next to the 12th-century All Saints' Church in Didcot. A post-mortem was performed Thursday.
Parkinson disappeared Dec. 3. Her boyfriend, Ben Blakeley, 22, of Reading has been charged with murder and perverting the course of justice, while a 17-year-old boy from Didcot faces the second charge.
Investigators said the results of toxicology tests would not be available for several weeks.
Didcot, with about 22,000 residents, is a substantial town 10 miles south of Oxford.