A mental health trust has launched an investigation after allegations staff dragged a heavily pregnant woman from a chair and pinned her to the floor.
One witness said the eight months pregnant woman was knocked face down by a male psychiatric nurse who worked for the Central and North West London NHS Trust.
Three staff members are thought to have been involved in the alleged incident on July 10 and one male nurse has since been suspended.
Former adviser to health secretary Jeremy Hunt, Alison Cameron, saw the events unfold.
She told ITV News: "I heard a patient shouting and swearing, which is completely normal for that ward.
"And the nurse in charge, it seemed to me, snapped and barged out the room. I was so concerned I followed behind him."
Earlier she had tweeted:
A statement issued by the Central and North West London NHS Trust said an investigation was now underway to establish exactly what happened.
It added: "We want to know why restraint was necessary, because it is always an extreme event, even when occasionally needed.
"After the incident a doctor examined the woman in question and she was fine.
"We are working with her and her family about the incident and helping her from here."