Deegan, 34, of White Shield, North Dakota, admitted Monday that she left the child alone for two weeks immediately after he was born, then put his body in a suitcase and left it in a ditch on the Fort Berthold Indian Reservation.
Deegan tearfully pleaded guilty in federal court to a reduced charge of second-degree murder, acknowledging that she was under stress around the time of the birth. She had originally been charged with first-degree murder and false statements.
She is to be sentenced in February and could face as many as 18 years in prison.
The suitcase containing the body was found in November 1999 by a rancher repairing fences. Prosecutors said the baby was found in a one-piece sleeper and socks, wrapped in a blanket and towel, and in a plastic bag.
The child became known as "Baby Moses," and community members paid burial expenses. Authorities eventually identified his mother through DNA testing. Deegan volunteered a DNA sample to the FBI in 2004.
U.S. Attorney Drew Wrigley said Monday that Deegan's plea helped bring justice for a baby whose life ended in starvation and dehydration.
The case is "absolutely gut-wrenching," said U.S. Attorney Drew Wrigley.
Deegan told Judge Daniel Hovland, mostly through a series of yes-or-no-questions, that she had never seen a doctor during the pregnancy and that she gave birth to the boy Oct. 20, 1998 .
She told Hovland that she gathered her older children - ages 1, 2 and 5 - and took them to her mother's home. She returned to her own home about two weeks later "by myself," she said.
Deegan said that neither she nor her children's father were employed at the time, and that they had no vehicle. The children's father used alcohol and was rarely around, she said. Her own father had cancer, she said.
Hovland asked Deegan whether her life had been "overwhelmed with stress," and she answered yes.
Deegan has been in her mother's custody. Defense attorney Bill Schmidt has declined to comment.