Lucy Letby bathed the body of one of her alleged victims in front of the baby's mother, her murder trial has heard.
The nurse, 32, is accused of murdering the boy, identified only as child E, and the attempted murder of his twin brother.
Their mother was meant to give birth at another hospital, but when the time came it did not have capacity.
As a result, she was transferred to the Countess of Chester Hospital, where Letby worked in the neonatal unit.
The nurse denies murdering seven babies and the attempted murders of 10 others while she worked at the unit between June 2015 and June 2016.
The twins' mother said that in the days after their birth, they seemed to be doing well, telling the court "my two boys were perfect".
One night when she came down to give them some of her milk she found Letby alone with them in the room and one of her sons crying.
"It was like nothing I'd ever heard before," she said. "I panicked. I was panicking. I felt something was wrong. That sound shouldn't come from a tiny baby, it was horrendous."
She walked over to the incubator and could see blood coming out of her son's mouth and asked Letby to explain what was going on, Manchester Crown Court was told.
Letby told her that the child's feeding tube was irritating his throat and instructed his mother to go back to her ward where she would be warned if she needed to come to the neonatal unit again.
'I trust her completely'
When prosecutors asked her why she did as the nurse instructed, she said that Letby was an authority figure.
"She knew more than me," she said. "I trusted her completely."
After the mother returned to her ward, she rang her husband because she still felt something was "very wrong".
Later that night, her child died.
She was later given the opportunity to bathe him, but said she couldn't bring herself to because she was "broken".
"Lucy Letby bathed him in front of me in the neonatal unit," she said.
The mother also revealed she was later given a teddy bear by Letby on behalf of the staff at the Countess of Chester.
The trial continues.