Police believe the baby of Constance Marten and Mark Gordon “may have been dead for some time” and have been unable to confirm its gender.
Marten, 35, and Gordon, 48, were arrested on suspicion of child neglect after a passer-by spotted them in Brighton.
They were re-arrested the following day on suspicion of gross negligence manslaughter and the remains of a baby were found on Wednesday.
The couple had been on the run since January 5 when their car was found abandoned and on fire next to the M61 in Bolton.
On Thursday morning, a cordon was in place at allotments which were at the centre of Wednesday’s search.
They were still being swept by officers from the Metropolitan Police, which is leading the investigation.
A forensic tent and police vans were in place in cordoned off Golf Drive while a nearby woodland area frequented by dog walkers was also closed to the public.
The pair remain in custody after police were granted a 36-hour custody extension by city magistrates on Wednesday.
Detective Superintendent Lewis Basford said the baby found in the Constance Marten and Mark Gordon investigation “may have been dead for some time” and the case has been referred to the police watchdog.
Speaking at a press conference on Thursday, the Metropolitan Police officer said: “At this stage we have not yet been able to confirm the baby’s gender and post-mortem examination has not yet taken place.
“Despite this, based on our inquiries we’ve carried out so far, we believe sadly the baby may have been dead for some time before they were found.
“It is too early for us to provide a more specific date.
“Because we believe the death occurred during the course of a missing person investigation we have made a mandatory referral to the Independent Office for Police Conduct (IOPC). This is standard protocol for such circumstances.”
He added that officers investigating the case were “devastated” by its outcome as he confirmed Constance Marten and Mark Gordon remain in custody.