"The woman is currently in custody helping officers with their enquiries."
The statement went on: "I would like to reassure people that we are doing everything we can as quickly as we can to identify what has led to these baby deaths and collapses.
"Due to the nature of this investigation it is extremely challenging but it is important to remember that it is very much active and ongoing – there are no set timescales and we remain committed to carrying out a thorough investigation as soon as possible.
"We fully appreciate that it continues to have a big impact on all those involved – including the families of the babies, staff and patients at the hospital as well as members of the public.
"Parents of all the babies are continuing to be kept fully updated and are being supported throughout the process by specially trained officers. This is an extremely difficult time for all the families and it is important to remember that, at the heart of this, there are a number of bereaved families seeking answers as to what happened to their children.
"At this stage, we are unable to go into any further details about the investigation but as soon as we can provide any further updates we will do.
The nurse worked at the hospital's neonatal unit since graduating from the University of Chester in 2011 with a degree in child nursing, and spent time at the unit during her three years training.
In July 2016, the Countess of Chester Hospital commissioned an independent review of the unit because of concerns about a rise in "unexplained" deaths.
