On March 4, police, criminal investigators and the ambulance service spent hours at an address in Gaer Vale, where a baby was found in poor health.
The baby was hospitalised on March 6 but died in the days following.
Police arrested a 35-year-old man on suspicion of child neglect. He has since been released under investigation.
A 30-year-old woman has now also attended a voluntary police interview.
The coroner has been informed and officers are continuing to investigate the circumstances that caused the baby’s death.
A spokesperson for Gwent Police said: “We received a report of a medical emergency at an address in Gaer Vale, Newport, at around 6.30am on Tuesday, March 4.
“Officers and paramedics attended, and a baby was taken to hospital. Sadly, the baby has since died.
“A 35-year-old man from Blaenavon was arrested on suspicion of child neglect. He has since been released on conditional bail.
“A 30-year-old woman has attended a voluntary attendee interview under police caution.
“HM Coroner has been informed, and our enquiries continue to determine the circumstances around the death.”