Altnagelvin Hospital in Derry has referred four cases of sudden infant death to a coroner for investigation.
The babies died over a number of months, the most recent being September.
The four died in the hospital's neo-natal unit, where sick or premature babies are treated.
A spokeswoman for the hospital confirmed the coroner's investigation began in September, and said the hospital was co-operating fully with the inquiry.
The ages and sex of the infants are not known and the spokeswoman said she was not in a position to provide further details.
She said any unexpected death is always a matter of sadness for staff and that the hospital had given its sincere condolences to the bereaved parents and families.
It is estimated that it could be up to a year before the coroner is in a position to deliver his report.
