In an extraordinary reversal, a Superior Court judge Friday rejected a jury's murder verdict against an Orange County, Calif., woman who drove a car over her infant son and instead found her not guilty by reason of insanity caused by a rare postpartum psychosis.

The decision by Judge Robert R. Fitzgerald in the case of Sheryl Lynn Massip stunned the Santa Ana courtroom, where cheers broke out from Massip's relatives and tears flowed freely. Prosecutors said they would appeal.The judge's ruling, which he later revealed he decided on at the last minute, means that Massip, 24, who faced life imprisonment after a jury last month found her guilty of murder, instead will go home with her family for Christmas. She faces the possibility of court-ordered psychiatric care, under which she could be committed to an institution for up to 11 years.

"I couldn't have asked God for a better Christmas present," said Massip, who had read the Bible while she awaited her sentencing in court. "I got the gift of life."

On her 23rd birthday, in April 1987, Massip tried to throw her 6-week-old son into oncoming traffic, then hit him over the head repeatedly with a blunt object and finally ran over him with the family Volvo.

Massip then told police her child had been snatched from her arms by a mystery woman with red hair and a gun. But shortly after Massip went to authorities, her son's tiny, battered body was found in a trash can.

Massip claimed she was suffering at the time from postpartum psychosis, a rare maternal disorder that is thought to spur severe anxiety, delusions and even violence in about three of every 1,000 new mothers. But an Orange County jury rejected Massip's claim, finding her guilty of second-degree murder.