By Peg McEntee

PROVO, Utah (Reuters) - A Utah mother pleaded guilty on Thursday to charges she murdered six of her newborn infants over the course of a decade and stuffed their bodies into cardboard boxes in the garage of her home.

Megan Huntsman, 39, who will be sentenced on April 20, had confessed to suffocating or strangling the babies while she was suffering from methamphetamine and alcohol addiction, police said. She faces a maximum term of five years to life in prison on each of the six counts of felony murder.

"We'd be surprised if she ever gets out of prison," District Attorney Jeff Buhman told reporters after the hearing, explaining that the state board of pardons was unlikely to grant her parole.

To his knowledge, Buhman said, Huntsman will be the only person serving time in the Utah State Prison, south of Salt Lake City, "who was responsible for six deaths."

Huntsman is not eligible for the death penalty because the murders, which took place between 1996 and 2006, predate changes in the law that would have made the offense a capital crime in Utah.

She entered her plea as she stood downcast in a Provo, Utah, courtroom, her wrists shackled to her waist, her head bowed, softly uttering "guilty" to each of the six counts.

The six infants' remains were found in April wrapped in old towels, shirts and plastic bags inside boxes in the garage in Pleasant Grove, a suburb just north of Provo. The body of a seventh infant was discovered disposed of in the same way but authorities have said they believe that child was stillborn.

Police have said Huntsman secretly gave birth to all seven without medical assistance at the house after apparently managing to conceal her pregnancies from the outside world.

Pleasant Grove police said the woman told detectives that she was hooked on methamphetamine and alcohol at the time, and killed the newborns because did not feel she could sustain her addictions while caring for the children.

The bizarre case of serial infanticide came to light when Huntsman's estranged husband Darren West - later confirmed by DNA tests to have fathered all of the victims - stumbled upon one of the tiny bodies while cleaning out the garage and notified authorities.

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Neither West nor any of Huntsman's three surviving daughters - aged 14, 18 and 20 at the time of her arrest - were considered suspects in the case. Those three daughters continued to live at the Pleasant Grove address, with other relatives, after their mother left them to move into a boyfriend's home in 2011.

At least one of the three, the youngest, was born during the period in which her sibling infants were slain. But authorities have offered no explanation for why she, too, was not killed.

(Reporting by Peg McEntee; Writing by Steve Gorman; Editing by Cynthia Johnston, Bill Trott and Lisa Lambert)