A pregnant woman is fighting for her life after being stabbed multiple times by a man she knew in Sutton Coldfield.
The 40-year-old victim was taken away from the scene by air ambulance at around 4pm.
A 41-year-old man was immediately detained in connection with the attack after the incident in the busy town centre, West Midlands Police said.
Speaking to reporters Chief Inspector Julian Harper said: "They are believed known to each other so this was what we call a domestic-related situation.
"As such no other members of the public should be alarmed and should be reassured that we have what we believe is the only person involved in this incident."
The investigation could become a murder probe if either the woman or baby dies, he said.
"Obviously if the lady or the baby should die it has that potential to be a murder inquiry. Obviously our wishes are with her and we hope that she will recover," he said.
Two members of the public - both men - suffered minor injuries after intervening and are being treated.
Mr Harper said initially one male intervened and then four younger males also came to help.
Anyone with information about the incident is asked to call police on 101 or Crimestoppers anonymously on 0800 555 111.
Mr Harper praised those who came to the woman's aid.
He said: "I would certainly like to give a big thanks to those individuals who have got involved, those members of the public who have dived into a volatile situation with complete disregard for their own safety and their actions should be commended."