Julie Alison Reynolds, 36, had six children ranging in age from 18 months to 17 years old by three different men. The newborn victim was her seventh child by a fourth man, said Deputy District Attorney Tracy Prior.
As part of her plea, Reynolds waived her right to have conjugal visits while she's behind bars.
"We don't want her having any more children while she's in prison," Prior told reporters outside court.
Reynolds' plea before Judge Robert F. O'Neill came on the day of her scheduled preliminary hearing. The judge set sentencing for July 10.
The defendant, who was originally charged with murder, was allowed to plead guilty to a lesser charge because she expressed remorse and accepted responsibility before a preliminary hearing was held, the prosecutor said.
Prior said it was still unclear why Reynolds smothered her baby hours after he was born.
"That's a $60 million question right there," the prosecutor said. "How could anybody take their child's life?"
At Reynolds' arraignment last summer, Prior said the defendant placed her left hand over the newborn's mouth, killing him. The child's body was found July 30 in a backpack in her bedroom closet.
Prior said Reynolds has a documented history with Child Protective Services in two states other than California.
The prosecutor said the defendant's sister and brother-in-law had welcomed the Colorado woman into their home a few months earlier so she could get her life back on track.
The couple didn't know Reynolds was pregnant, but noticed some weight gain and repeatedly asked the defendant if she was with child, but Reynolds denied it, Prior said.
The weekend before the killing, the couple became extremely suspicious when they noticed Reynolds' weight loss and found blood on the carpet in her bedroom, the prosecutor said.
The brother-in-law went through the trash and found a plastic bag with bloody towels, then went to the police for help, Prior said.
When officers arrived, they found the 6-pound, 19-inch baby in the backpack, the prosecutor said.
Prior said the defendant knew she had the option of dropping the newborn off at a police or fire station, having her family help or giving the child up for adoption, but chose none of the options.
The defendant's six children are in protective custody and her relatives offered to take care of them, Prior said.
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