The girl, whose mother's body was found Monday in a closet at her Worcester apartment, appeared to be in "fairly good health" at a New Hampshire hospital, Worcester Police Sgt. Kerry Hazelhurst said late Wednesday.
Julie Corey, 35, and a male companion were arrested in Plymouth, New Hampshire, where police found them with the child. Friends had told police they were suspicious of Corey's claims she had given birth.
Corey was charged as a fugitive from justice. She was in custody and could not be reached for comment late Wednesday. Police did not know whether she had a lawyer.
The baby's mother, Darlene Haynes, was eight months pregnant. Haynes' body was found by her landlord, William Thompson, who said a "horrifying smell" led him to her apartment, where he found her body wrapped in bedding in a closet. Her death was ruled a homicide.
Police said the 23-year-old had apparently been dead for several days, and that she hadn't contacted family or friends since Thursday. It was not until an autopsy Tuesday that authorities discovered the fetus was missing.
Corey, who police said had reportedly gone to New Hampshire to relocate, arrived at a Plymouth homeless shelter Tuesday night. She told workers there that the girl was 6 days old and identified herself as the mother, but had no information on the child, according to the Union Leader newspaper in New Hampshire.
On "The Early Show" Thursday, CBS News medical correspondent Dr. Jennifer Ashton said the baby, reportedly 36 weeks old, is being watched closely in the hospital, but even under the circumstances, she said the premature infant can survive well outside the mother.
"Babies are resilient," Ashton said. "This shows us that."
Corey was arrested Wednesday afternoon as she tried to leave the shelter with the infant after workers alerted police and a nurse began photographing the baby with her cell phone, the newspaper reported.
The exact cause of Haynes' death has not yet been determined pending toxicology tests, but Worcester Police Detective Capt. Edward J. McGinn Jr. said the autopsy indicated Haynes suffered head injuries.