A gang of masked men broke into a Feltham home demanding gold and threatening the occupants including a heavily pregnant woman.
The men, wearing dark clothing and balaclavas, broke into a house in Cains Lane by smashing the rear door of the home.
One of the six men involved in the break in, described as having an "Eastern European" demanded to know "where is the gold?", the Metropolitan Police said.
The suspects left empty-handed but poured or sprayed a "chemical substance" by the front door to prevent them being chased, the police said.
None of the occupants were injured in the incident at 2.40am on Thursday (September 14).
The suspects made their escape in a white van, heading in the direction of Clockhouse Roundabout via Hatton Road and Staines Road.
PC Richard Harris, from Hounslow Police, said: "This was a very traumatic experience for those in the house.
"They were woken in the early hours and threatened by the suspects.
"I would urge anyone with information that could assist this investigation to contact police."
Police have described one of the suspects as 5ft 8in and of stocky build with brown eyes.
He was wearing a black balaclava, a black jacket and trousers and was believed to be carrying a crowbar.
Police are also appealing for information about a second suspect, also 5ft 8in.
There have been no arrests and enquires continue.
Anyone with information is asked to contact officers at Hounslow on 101 or Crimestoppers anonymously on 0800 555 111.
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