The nurses both work at the Princess Alexandra Hospital in Harlow, Essex.
Detectives were called in after a 10-day-old boy suffered a broken leg on July 14 while in the neo-natal intensive care unit. It is understood officers are looking at other incidents in which premature babies have suffered injuries while at the unit. Essex Police yesterday confirmed an investigation was under way.
Sally Gorham, chief executive of the Essex & Herts Community NHS Trust, said two nurses at the hospital had been suspended and an internal investigation was also under way.