The Alberta Court of Appeal said Katrina Effert's second-degree murder conviction wasn't warranted, the Canadian Broadcasting Corp. reported.
Effert had been facing a life sentence with no chance fof parole for 10 years but can receive no more than five years on the lesser count. Her case was remanded to a lower court for sentencing.
Effert was 19 on April 13, 2005, when she secretly gave birth in her parents' Wetaskiwin home, strangled the baby boy with her underwear and threw the body over a fence into a neighbor's yard.
A jury in Wetaskiwin convicted her in 2006 but that verdict was overturned on appeal. A second jury in Edmonton reached the same verdict in 2009.
In overturning the murder conviction, the appellate court said juorsthe jury should have given more weight to testimony by two clinical experts who said Effert was mentally disturbed at the time of the killing, the CBC said.
The appeals court said overturning a jury's verdict must not be done cavalierly.
"Juries are an important part of the criminal justice system and the integrity of their verdicts must be protected," the court said. "There are, however, cases where the verdict of the jury cannot be sustained.
"Where a verdict is unreasonable, in the sense that it cannot be supported by the evidence before the jury, then the jury must have made an error in its analysis or failed to understand the instructions given."
The court said it wasn't known why the jury chose to convict but speculated, "Perhaps this jury was distracted by the tragic circumstance of the death of a newborn infant."