The infant, less than 2 months old, was found dead in his crib -- a victim of Sudden Infant Death Syndrome, the medical examiner said. Within months, the case was forgotten by all but the grieving relatives.

Then in June, three years after young William Porritt Jr. died in Fairfax County, his family's anguish reached deeper. The mother confessed, first to a priest and then to her father-in-law, that the child was a victim not of SIDS, but of murder.

Yesterday, the secret tragedy of William Porritt Jr.'s death came to trial in Fairfax County Circuit Court, where the mother, Carla J. Porritt, pleaded guilty to first-degree murder. Porritt, who remains married to the child's father and is pregnant with another child, admitted suffocating her son 52 days after he was born.

Porritt could face up to life in prison if she is not sentenced to serve in an alternative program. Sentencing is set for Dec. 7.

The baby died on May 30, 1987, according to court records. Porritt, 30, of the 400 block of N. Armistead Street in Alexandria, kept the secret for three years until she broke her silence in June.

The child's grandfather called Fairfax police after Porritt telephoned him in Orlando, Fla., to tell him she killed the child and to apologize, according to Commonwealth's Attorney Robert F. Horan Jr.

"He told them that his daughter-in-law, Carla Porritt, recently had contacted him by phone and told him that she had killed her child," Horan said.

Porritt, who according to court documents, functions on the borderline of mental retardation, was arrested June 19 after she gave a statement to police and a videotaped demonstration of how her son had been suffocated.

Horan played the tape for Judge William G. Plummer during yesterday's hearing. On a grainy screen, Porritt sat behind a table with her hair pulled into a ponytail and answered questions from police investigators.

"Why did you want to kill the baby?" Investigator T.J. Lyons asked.

"Because I was jealous and I was tired of it and plus I was, well, that would come under lazy, you know," she told police. "You know, I did fine at first, you know. I should of stuck with it, you know. This is very wrong and dangerous, but at least I surrendered to it, you know, because I know that since it was wrong, I know this is, this is something I've never done before, even in my suffering."

She later said that she was jealous of the attention that her husband, William Porritt Sr., gave the baby. "I wanted Billy all to myself," she told police.

Lyons asked Porritt how she killed her son. She pulled a baby doll from beneath a table and demonstrated how she pinched the boy's nose, covered his mouth and waited. She stopped, but his heart was still beating, Porritt said. Then she did it again.

"I was determined, you know, to try to kill him, you know," she said.

Porritt said she waited "a little while to call 911" because "I thought I could, you know, get away with it."

Horan said after the hearing that he thought it was "the most pathetic case" he had seen in his 25 years of prosecuting.

Despite Porritt's limited intelligence, the state found that she was sane at the time of the slaying and was competent to stand trial. "She obviously has some serious mental deficits," Horan said. "The question in a case like this is what do you do with it.

"She commits the ultimate crime. There is no worse crime than killing one of your own," Horan said. "But at the same time she does not fit the standard of quote, unquote a criminal."

Public Defender R. Dean Kidwell said yesterday that the case was tragic, not only in the death of the infant but in what it has wrought in the aftermath.

Kidwell agreed with the state's assessment that Porritt was legally competent to stand trial. But Kidwell said that outside the legal realm, Porritt clearly is incompetent to function on her own.

Porritt's sister, Paula Russo, of Fairfax, said yesterday that she is outraged by the court system's finding that her sister was competent to stand trial. Russo said her sister, one of two retarded siblings, has always been able to fool people on her intelligence.

"Carla has a long history of mental retardation," Russo said. "It's so borderline, it's hard to see sometimes."

Russo said she believes Porritt knows the difference between right and wrong, but that she doesn't know the extent of how wrong it was to have killed her baby.

"She doesn't understand the full repercussion of her actions here," Russo said. "It's like dealing with small pets. Oftentimes people will buy small pets. They get bored with it and neglect it and the animal dies as a result. That's the same theory Carla is using. She doesn't understand there was a human being she destroyed."

Russo said Porritt is the youngest in a family of four children. Carla completed the 12th grade, studying in special education classes. After she married in the early 1980s, her first daughter was taken away by the county Department of Social Services, which found that neither she nor her husband were fit to be parents, her sister said.

Russo said Porritt grieved over the loss of her daughter, Christina, who was later adopted. Soon after, Porritt left her first husband and went to live in a boarding house in Baileys Crossroads. She later married William J. Porritt Sr. and had William Jr.

William Porritt Sr. said yesterday after the hearing that Carla beat her stomach during the pregnancy. And when the baby was born, she was unhappy with the attention he showed the infant.

"He was good-looking," he said. "He almost looked like me, close to me, when I was a baby."

Even after watching the videotape, William Porritt said, he did not believe his wife had killed the child. "The whole thing is driving me crazy," he said.

Carla Porritt, who is eight months pregnant, sat in court yesterday bobbing her head. She turned to her husband, who sat in the third row, and smiled, then swung around to face the judge. When she answered Plummer's questions, she spoke in a strong voice.

"I'm guilty, your honor. I'm here to tell you the truth," she said. "I'm doing the right and best thing for me, to step forward and confess."

Porritt is being held in the Fairfax County jail.