A second hospital is co-operating with a police inquiry into the deaths of 17 babies at the Countess of Chester Hospital's neonatal unit.
Lucy Letby, 28, continues to be questioned in custody on suspicion of the murder of eight babies and the attempted murder of six other infants.
Officers arrived at the nurse's home in Chester on Tuesday and several hours later police announced a female healthcare professional had been arrested as part of the "highly complex and very sensitive investigation".
Police are said to have arrived at Ms Letby's home at around 6am, while officers were also seen at her parents' property in Hereford later on Tuesday.
It is understood that Ms Letby worked at the Countess of Chester after she graduated as a children's nurse from the University of Chester in 2011 and went on to complete a work placement at Liverpool Women's Hospital as part of extra training.
On Wednesday, Liverpool Women's NHS Foundation Trust said in a statement: "A healthcare worker currently involved in a police investigation undertook placements at Liverpool Women's during their training.
"We are co-operating with police as part of their investigation which includes a routine review of patients cared for on our neonatal unit during the time of these placements.
"There is currently no suggestion that any patients at Liverpool Women's came to any harm in relation to this investigation."
A blue tent erected by police remained outside the front of Ms Letby's semi-detached property in Westbourne Road, Blacon, with a uniformed officer stationed nearby.
Detectives launched an investigation into infant deaths at the hospital in May last year, initially looking at the deaths of 15 babies between June 2015 and June 2016.
On Tuesday police said the probe had widened to include 17 deaths and 15 non-fatal collapses of babies between March 2015 and July 2016.
Detective Inspector Paul Hughes said the arrest was a "significant step forward" in what was a "highly complex and very sensitive investigation".
He added that the probe was "very much active and ongoing" and that parents of all the babies were being updated on the inquiry and supported by trained officers.
Police did not confirm the identity of the arrested health worker.
The Countess of Chester Hospital carried out a number of independent expert medical reviews into the deaths before calling in police.
Medical director Ian Harvey said officials were continuing to support officers with their investigation.
He said: "Asking the police to look into this was not something we did lightly, but we need to do everything we can to understand what has happened here and get the answers we and the families so desperately want."
Mr Harvey added that the hospital is "confident the unit is safe to continue in its current form".
A report published in July 2016 by the Royal College of Paediatrics and Child Health (RCPCH) found significant gaps in medical and nursing rotas and insufficient staffing for the provision of longer-term, high-dependency and some intensive care.
The RCPCH report said paediatric consultants had been concerned since June 2015 about a higher than usual number of deaths on the ward, several of which were "unexplained" or "unexpected".
In a 2013 interview with the Chester and District Standard, Ms Letby said she cared for babies requiring various levels of support.
She had worked at the unit as a student nurse during three years of training before qualifying as a children's nurse at the University of Chester in 2011.
She said she started working at the unit after graduating.