Jordan Brown, now 12, is charged with criminal homicide in the death of 26-year-old Kenzie Marie Houk in their farmhouse in western Pennsylvania.
Ms Houk was eight-and-a-half months pregnant when Brown allegedly shot her in the back of the head as she lay in bed on February 20, 2009. Her unborn child - a boy - suffocated after her death.
Police had said Brown, now 12, hid the weapon under a blanket so her seven-year-old daughter would not see it as he entered her mother's room.
Brown then took the bus to school, apparently throwing the shotgun shell casing out on the way. A state trooper testified that tests showed the shell was fired from Brown's youth-model 20-gauge shotgun, the gun he is believed to have received as a Christmas present.
Houk's body was only found when her four-year-old daughter told workers outside the house that she thought her mother was dead.
Prosecutors claimed Brown had been jealous of his father's new family.