Nurses working with Lucy Letby began to speculate about whether someone could be harming babies nine months before the serial killer was taken off frontline duties, a public inquiry heard.
Dr Rachel Lambie, a former paediatric registrar at the Countess of Chester Hospital, said some nurses had been "starting to think the unthinkable" as early as September 2015.
Letby was finally moved from the hospital's neonatal unit on to clerical duties in July 2016, and was first arrested in 2018.
The Thirlwall Inquiry, which is examining how the NHS handled the period, heard how a series of unexpected deaths and collapses had led to Dr Lambie always "expecting something bad to happen" at the start of her shifts.
She was asked about unexpected deteriorations among babies in the period she had worked in Chester, between April and September 2015.
The consultant told the inquiry, sitting at Liverpool Town Hall, that in September that year she had seen a group of nurses huddled over a computer in the intensive care unit.
'Awful thing to say'
Dr Lambie said she had been told they were going through the rotas to see if someone had been on shift for each of the unexpected deaths and collapses.
She said one of the nurses had remarked: “It’s an awful thing to say but we’re just looking.”
Dr Lambie told the inquiry: “It is the unthinkable but that’s what they were discussing.
"They were starting to think the unthinkable, that there might be a person causing deliberate harm.
"It was starting to become part of their thought processes."
Dr Lambie told the inquiry that during the time of the unexpected events she “was nervous at the start of a shift because I’m expecting something bad to happen.”
She said she had had a “heart-sink feeling" and thought "oh God, what’s going to happen today?".
Dr Lambie said when twins referred to as Babies A and B collapsed in June 2015, there had been anxiety among doctors because of an unusual rash seen on both children.
Letby was convicted of murdering one of the babies by injecting him with air and of attempting to murder his twin sister.
The doctor said she might support the idea of medical staff wearing cameras that could be turned on and off in a neonatal unit if that would help protect patient security and safety.
But she thought having fixed CCTV cameras would be difficult due to privacy concerns, with such units hosting new mothers who were often breastfeeding.
Letby was convicted of murdering four babies and trying to murder two others during the period Dr Lambie worked at the hospital.
Overall, she was convicted of murdering seven babies and attempting to murder seven others between June 2015 and June 2016, including two attempts to kill the same baby.
The 34-year-old, originally from Hereford, is serving 15 whole-life prison terms.
The inquiry continues.