The first child is due of a missing man who police presume has been murdered.
The Met Police said they have now arrested a fifth man in connection with their investigation into Mohammed Shah Subhani, 27, who disappeared on 7 May.
Known as Shah, he was reported as missing after failing to return to his home in Hounslow, west London.
Det Ch Insp Noel McHugh said: “Shah’s first child is due to be born any day now and this should be a joyous time for his family.”
He added: “Instead their lives are tormented with fear and uncertainty.”
On Thursday, a 19-year-old-man was the fifth person arrested at an address in west London on suspicion of conspiracy to commit murder and kidnap, police said.
The Met have also interviewed a man under caution at a London prison, where he is serving time for an unrelated matter.
Another development is the recovery on 3 August, in Hounslow, of a stolen black BMW X5 that was on cloned plates, YC67MFY.
“We are also searching two addresses, in west London, for evidence that may lead us to those who abducted Shah.
“In addition, the seizure of the BMW X5, which was found on false plates in Hounslow, is another significant development.”
The Met previously said Mr Subhani may have become “out of his depth in some kind of criminal activity”.
Mr Subhani’s car, an Audi Q3, was recovered with ballistic damage on 19 June and this, taken with the circumstances of his disappearance, means police are now treating the case as a murder inquiry.
A reward of £20,000 remains on offer for information that leads to the arrest and conviction of those involved, police said.
Four men have previously been arrested in connection with the case.
Three have been released pending investigation and one remains on bail, police said.