A Fairfax County teenager who left her newborn baby in the bathroom of an Arlington church said yesterday that she told no one about the pregnancy and gave birth by herself in her bedroom after doing research at the library and on the Internet.

Arlington authorities announced yesterday that they will not file charges against the 17-year-old in the June 11 incident. Commonwealth's Attorney Richard E. Trodden said the girl's actions did not rise to the level of criminal negligence. Because the baby was in the bathroom at St. Agnes Catholic Church for only a short time, Trodden said, it would have been difficult for prosecutors to show "wanton and reckless disregard for human life," as required by state law.

The teenager, who is a high school student and a member of St. Agnes, told police that she did not abandon her baby girl but merely left her in the bathroom while she went to look for the pastor. She said that when someone else at the church discovered the baby only a few minutes later, she was too frightened to acknowledge that the child was hers. Police said they learned the mother's identity five days later.

The teenager, who agreed to an interview yesterday on the condition that she not be identified, said she was elated to learn that she will not be prosecuted and is hopeful she will get to keep her baby.

"I was extremely excited," said the mother, who has been allowed to visit her daughter several times. "Now I'm just hoping my baby comes home soon. She's eight pounds now. She's growing fast now. She's just so precious."

Both Trodden and Cpl. Justin McNaull, an Arlington police spokesman, declined to comment on whether authorities believe the teenager's version of events. McNaull stressed that the baby was in "real medical distress" when rescue workers arrived at St. Agnes shortly after the infant was discovered.

"For five days, this story and the baby's picture were on the television and in the papers, and this lady did not come forward," McNaull said. "It was not until we went to her house and her school that she initiated contact with us through her lawyer."

Her attorney, William G. McMurtrie, said the absence of a criminal charge will improve the teenager's chances of keeping the baby, who is being cared for in a foster home.

The teenager said that when she learned she was pregnant, she decided she would keep the baby "no matter what." She said she did not receive any prenatal care and decided to learn about pregnancy in the same way that she would tackle a school assignment, by studying books and Web sites.

"I had read up on it all throughout my pregnancy," the teenager said. "How the baby grows inside. Things you should eat. The do's and don'ts. I had also read up on emergency labor."

When her water broke, she said, she sterilized scissors to cut the umbilical cord and covered her bed with a plastic cover and extra sheets.

Her labor lasted about nine or 10 hours, the teenager said. Her parents were at work, and she wasn't scared because "I was basically prepared for what was going to happen," she said.

Arlington officials said that the girl was lucky she had a successful birth and that no one should try to deliver a baby on her own. "This isn't the way we would want any baby to enter the world or spend the first few hours of its life," McNaull said.

After the birth, the teenager said, she took the baby to her car and drove around for a few hours, finally deciding that she would confide in the pastor at St. Agnes, the Rev. James R. Gould.

Because it was hot outside, she took the baby into the cool basement bathroom of the church and left her there to wait for Gould to finish Holy Hour services, she said. As it turned out, Gould had not conducted the service. She said that when she went back to the bathroom to get her baby and go look for him, she saw that another woman was there and she panicked as her baby was carried to the church office.

"I didn't know what to do," she said. "I had not planned to be seen by anyone other than Father Gould."

She said she left the church and did not contact police over the next few days because she was worried she would get in trouble. Not until police called her, after receiving an anonymous tip, did she finally tell her parents, she said.

The teenager said her parents will help her raise the child if she gets to keep her. After graduating from high school, the teenager will attend a college in the area to stay close to her daughter, she said.

She said she plans to raise her daughter as Mary Agnes, the name another pastor at the church used when he baptized her before the infant was taken to the hospital.

"It fits her so perfectly," the teenager said.