She was pronounced dead at the scene, according to the Oklahoma Department of Corrections.
Hilberling was serving a 25-year sentence after being convicted of second-degree murder in 2013 for the death of her husband, Josh Hilberling, an Air Force veteran, who died after falling from a 25th floor window.
Hilberling admitted to pushing her husband, but claimed self-defense, saying the couple was having a fight and she feared for her life.
She was seven months pregnant with the couple's first child at the time of the incident, and said she did not intend to kill him.
But prosecutors argued that, even if Amber did not intend to cause Josh's death, pushing him towards the window showed a dangerous disregard for his life.