Police have urged parents to be vigilant after a bogus social worker called at a house and examined a baby.
The woman, who claimed she was from Gloucestershire social services, was carrying a false ID badge and a black zip-up folder.
She visited a mother in Quedgeley, Gloucestershire, and told her there were concerns for the welfare of her four-month-old son.
The fake social worker asked to carry out checks and listened to the boy's heartbeat with a stethoscope during the incident on Wednesday.
Detective Inspector Andy Dangerfield, of Gloucestershire Police, said the woman did not have any physical contact with the baby.
"We don't know what the motivation for this was but clearly it is very concerning," Mr Dangerfield said.
Police have appealed for information about the woman, who is described as white with "slightly tanned skin", in her late 20s or early 30s and with dark shoulder-length bobbed hair.