The boy removed a handgun from his mother's purse yesterday and fired one shot, striking his father in the lower backside in a hotel room in Albuquerque.
The bullet apparently exited through the father's hip and hit the mother, who was eight months pregnant, in the right shoulder, the CNN reported.
The father has been discharged from the hospital while the mother is still hospitalised.
The boy and his 2-year-old sister, who was also in the hotel room, are under the care of the children welfare authorities.
More From This Section
The incident is the latest in a series of shootings in the US involving children.
On January 21, a 2-year-old boy accidentally shot and killed himself with his father's gun in a car in Florida.
Earlier, a 9-month-old boy was killed after his 5-year-old brother playing with a handgun accidentally shot him in the head in Missouri.
Another 4-year-old girl killed her cousin in Detroit while playing with a gun found under a bed earlier this month.