A baby was last night in a critical condition as detectives continued to quiz the child's mother.
The infant was rushed to hospital following an incident in east Belfast.
The child is being treated at the Royal Hospital for Sick Children in Belfast and is fighting for its life.
Forensic officers continued to examine the property at Connsbrook Avenue yesterday where the infant was found on Friday evening.
A cordon remained in place throughout the weekend.
Neighbours were shocked to learn of the incident but were unaware of the identity of the woman arrested.
One said he believed foreign nationals had been living in the property recently.
East Belfast MLA Michael Copeland said he was saddened by the news.
"You never know the tragedy that is behind any door," he said.
"The circumstances surrounding this particular case remain unclear, and early speculation is neither helpful or necessary."
UUP councillor Jim Rodgers added: "This sounds absolutely terrible and I hope the child makes a full and speedy recovery.
"The circumstances are still not clear but it is very concerning."
Police declined to reveal further details yesterday.
A spokesman said: "Police attended the scene of an incident in the Connsbrook Avenue area of east Belfast on Friday.
"An investigation is under way and a woman has been arrested in connection with the incident."