Professor Bernard Knight spent more than two hours examining the remains discovered on Tuesday in the Fingerpost Field at Kempley on the Gloucestershire border with Hereford and Worcester.
They were found at the edge of a pit, carved out over the past eight weeks by specialist diggers in the murder investigation, which has already recovered 11 sets of human remains elsewhere.
The digging stopped to allow Professor Knight to make a detailed examination of the grave site, five feet below the surface in heavy clay.
He said: 'This is a young woman in her late teens or early twenties and in a fairly advanced state of pregnancy.' The remains were taken to Cardiff for examination. Police have declined to comment on speculation that the remains are those of Anna McFall, a former nanny to the children of Frederick West, 52. Miss McFall, from Coatbridge, Lanarkshire, was about 22 when she disappeared in the early 1970s.
The new find was made only a short distance from a grave in the neighbouring Letterbox Field, where police recovered the remains of a woman, assumed to be those of Mr West's first wife, Catherine. These remains are yet to be formally identified.
Mr West and his wife Rosemary, 40, are jointly accused of the murders of nine young women and girls whose remains were recovered from their house and garden at 25 Cromwell Street.
They are due to appear before Gloucester magistrates on 30 June. Mr West faces 11 murder charges.