ABC19980309.1830.0519 NEWS CAPTION A teenager who was charged along with his former high school sweetheart with killing their newborn son has agreed to cooperate with the prosecution. Today Brian peterson pleaded guilty to manslaughter, which carries a possible ten-year prison sentence, and he has agrowed to testify for the prosecution at amy grossberg 's, his girlfriend's, murder trial. by the time Brian peterson walked into court this morning, he had made his choice -- plead guilty to avoid being tried for murder. Standing next to his client, peterson's attorney told the court the 19-year-old believed the baby was born dead and that he quickly put the baby in a plastic bag because amy grossberg told him, "get rid of it. Get rid of it." He did, dumping the baby in this motel trash bin. In his plea agreement, peterson promised to testify in the murder trial of his former girlfriend in may. Brian is not anyone's adversary. His duty is to testify truthfully if he is asked to do so. but grossberg may have become peterson's adversary. Recent pretrail motions by grossberg's attorneys made peterson fear grossberg would testify she thought she miscarried and tried to put all the blame for the baby's death on him. Initially, the high school sweethearts remained loyal. As recently as last June, grossberg told Barbara walters she and peterson would never turn against one another. Brian and I have a really good relationship. I care for him a lot. He's a great kid. And that will always -- that will stay the same. but now, their relationship Seems irrelevant when compared to the need to save themselves from the threat death sentences for murder. Antonio Mora, ABC News, wilmington, Delaware.