NEW YORK — A woman who snatched a newborn from a hospital more than two decades ago and raised the child as her own was sentenced Monday to 12 years in prison by a judge who called it a ‘‘crime of selfishness’’ that alarmed parents and forced hospitals to boost security.

Ann Pettway, of Raleigh, N.C., nodded her head repeatedly as US District Judge Kevin P. Castel announced his sentence in a Manhattan courtroom packed with relatives and friends of Pettway as well as the true parents of Carlina White, who was stolen from Harlem Hospital when she was only 19 days old.

Pettway, 50, had pleaded guilty to kidnapping in February, describing how she took a train in 1987 from her Connecticut home to the hospital, where she posed as a nurse and reassured White’s mother, who had brought her daughter to the emergency room for treatment of a high fever.

The sentence was far less than the 20 years in prison recommended by prosecutors and the Probation Department, but it was within the 10 to 12½ years agreed to by prosecutors and defense lawyers in a plea agreement.

Castel scolded Pettway, telling her she had ‘‘inflicted a parents’ worst nightmare on a young couple,’’ caused other parents to fear child kidnappings, and led hospitals to install preventive measures to avoid similar kidnappings. He said it was ‘‘not a crime of greed, of vengeance. But it was an act of selfishness, a crime of selfishness.’’

Pettway apologized, saying she was ‘‘deeply sorry for what I have done,’’ and sought to ‘‘right my wrong and to ask for forgiveness.’’

Carl Tyson, the biological father, glared at Pettway, saying: ‘‘You hurt me really bad.’’

Joy White, the mother, said her daughter had found her but: ‘‘I still haven’t found my daughter.’’ She said she ‘‘can’t change how my daughter feels’’ when she talks about Pettway’s relatives as if they are her own.

‘‘My daughter is here, but she’s not home yet,’’ she said.

Outside court, Tyson said the sentence ‘‘should have been longer,’’ but White said she was satisfied. ‘‘If they gave her 200 years, it’s still not going to make up for the time my daughter was gone,’’ she said.

Assistant US Attorney Andrea L. Surratt said Pettway told ‘‘complicated lies’’ for 23 years. In court papers, she challenged Pettway’s lawyers’ contention that she provided a ‘‘stable, loving, and happy home,’’ saying Pettway was convicted of five crimes while the child was in her custody and had confessed to cocaine use from 1983 through 2005.

In a letter to the judge, Pettway apologized, blaming untreated psychological disorders from her failed pregnancies.