Constance Marten and Mark Gordon have been further arrested on suspicion of gross negligence manslaughter as police search for their missing baby.
Police say they must now consider the possibility the infant “has come to harm”.
But they say that they still hope to find the baby safe and well.
The couple were initially arrested on Monday on suspicion of child neglect after being missing for 53 days.
Detective Superintendent Lewis Basford, from the Metropolitan Police, said that risk to the baby increased as time went on, especially amid cold weather.
He added that police must now “be open to the fact this may not end in the way we would like”.
Det Supt Basford told journalists that little information had been gathered during police interviews with the couple and that police did not yet know the sex of their baby.
Police believe Ms Marten and Gordon have been “in open land and open areas” and say they are searching a 91-square-mile area in Brighton for their child.
Ms Marten and Gordon, a convicted sex offender, were found after being spotted by a member of the public in the Fiveways area of north Brighton on Monday evening.
More than 200 officers have been deployed to Sussex as part of the investigation, police said. Local residents in Stanmer Villas have been asked to check sheds and outbuildings to assist the search.
Temperatures are expected to plummet to 1C overnight in the Brighton area.
Police have not ruled out that someone could be harbouring the baby, though they said this was unlikely.
A police investigation into Ms Marten and Gordon’s whereabouts began in early January, after the couple’s car was found burning at the side of a motorway near Bolton.
Police say the couple safely left the scene of the fire and travelled to Liverpool, Essex, London and East Sussex in quick succession. Before they were found on Monday, the last confirmed sighting of them was in Newhaven, East Sussex, on 8 January.
At the end of January, police offered a £10,000 reward for information that led to the couple and the baby being found safe.
The couple’s home is in Eltham, in south-east London, but they have been living nomadically since September last year, when Ms Marten first started to show signs of pregnancy.
After the pair were found on Monday, Ms Marten’s father Napier Marten told the Independent he felt “immense relief”, though this was “tempered by the very alarming news [Ms Marten’s] baby has yet to be found”.
Ms Marten, 35, is from a privileged background, having lived in a stately home growing up and attended private school. Police previously said her inherited wealth may have allowed the couple to remain at large for an extended period.
She became estranged from her family after meeting 48-year-old Gordon at drama school in 2016.