Police have arrested missing aristocrat Constance Marten and her partner Mark Gordon in Brighton after going on the run for almost eight weeks.
Their baby has not been found, said the Metropolitan Police.
The pair were located by officers from Sussex Police in Stanmer Villas, Brighton on Monday night after a member of the public reported seeing them shortly before 9.30pm.
Ms Marten, 35, and her partner have been travelling around the UK by taxi since their car was found burning on the M61 in Bolton, Greater Manchester, on 5 January.
The couple went from Bolton to Liverpool, then to Harwich in Essex, then to east London and then to Newhaven in Sussex, where they were spotted near the ferry port on 8 January.
Ms Marten, an heiress to a wealthy aristocratic family, was a promising drama student when she first met her partner in 2016.
Since then the couple have led an isolated life, and in September, when she was well into her pregnancy, they began moving around rental flats.
A registered sex offender, Gordon served 20 years in prison in the US for rape and battery committed when he was 14.
On the search for the baby, the sex of which is not known yet, detective Superintendant Lewis Basford from the Metropolitan Police said this morning: “It is a vast area.
“We are looking at the local vicinity to where the couple were arrested last night.”
Asked whether the couple had said where the baby is, Det Supt Basford said: “At this time we’ve got no information from the inquiries we’ve had since the arrest that are leading us to any locations where the baby is.
“We’re certainly working on the premise that the arrest location was close to open land and the couple moving towards that land and that’s why we’re focusing heavily on where we are right now.
“They [Marten and Gordon] have been arrested on suspicion of child neglect and there is a search for the baby.
“We are looking at open land, outbuildings where they may have placed the baby while they were in the local area before we arrested them.”
Last week the director of midwifery for Barts Health NHS Trust issued a statement urging the mother to return home so the baby could receive medical attention.
“I know that you really love your baby, and I know what an exciting and worrying time being a mum can be. And I want to make sure that you and your baby are OK and get the care you need,” said Shereen Nimmo.
“After you have a baby, midwives are there to check you are recovering physically and emotionally and adjusting to caring for your newborn.
“We check your baby is feeding properly and putting on weight, and they have screening tests to make sure their heart, hearing, hips and eye sight all look OK.
“We also check for some rare conditions and put care plans in place to support them if needed.
“You’re putting your baby at risk by not accessing medical care, so it’s really important that you come and see a midwife, doctor or another healthcare professional as soon as possible.”
Earlier this month, Ms Marten’s mother also issued an open letter saying she wanted to “help” her daughter.
On 19 January, Ms Marten’s father Napier Marten issued an appeal to his estranged daughter in The Independent.
“I want you to understand that you are much much loved whatever the circumstances. We are deeply concerned for your and your baby’s welfare”.
Mr Marten, 63, told The Independent he had known about Gordon’s criminal conviction for some time.
He said: “Darling Constance even though we remain estranged at the moment, I stand by, as I have always done and as the family has always done, to do whatever is necessary for your safe return to us.
“I beseech you to find a way to turn yourself and your wee one in to the police as soon as possible, so you and he or she can be protected. Only then can a process of healing and recovery begin, however long it may take, however difficult it may be.”
