The sterile, cheerless environment of the D.C. Jail was brightened unexpectedly yesterday by a "first" -- the birth of a healthy, 6-pound by to an inmate.

A 26-year-old woman gave birth unassisted in her cell in what jail officials said was the first time a baby had been born inside the jail.Other inmates have given birth while confined there, but all were transferred in time for the births to occur at nearby D.C. General Hospital.

Mother and son were reported in good conidition at D.C. General, corrections department official Leroy Anderson said. Anderson refused to release the mother's name, saying that do to so would invade her privacy.

Anderson said the woman gave birth at around 4:45 a.m. "She called out, and when the officers arrived they found her on the bed with the baby," he said.

Anderson said the mother had been jailed on March 13 for walking away from a halfway house, where she was being held for unauthoried use of a vehicle. She would have been eligible for parole sometime in July, Anderson said.

He said the woman, who is unmarried, was taken to D.C. General on June 3 for a prenatal checkup and was told by doctors that she was seven months pregnant.

If that deiagnosis was correct, the baby was born prematurely. Anderson said of the 6-pound baby boy, "That's a pretty big 'preemie.'"