The needed tests could take a couple of weeks as police continue to look into the macabre case.
Preliminary reports from detectives were that the fetus was born alive and possibly had been asphyxiated, but chief New York Police Department spokesman John McCarthy said that the case was still being investigated and that police were awaiting the medical examiners' determination of the cause of death.
The case began Thursday when a security guard stopped Tiana Rodriguez and Francis Estevez, both 17, to examine their bags at a Victoria's Secret store in midtown Manhattan. The guard found the dead fetus in a bag Rodriguez was carrying, police said.
Rodriguez told detectives she was carrying the remains because she had delivered a day earlier and didn't know what to do, authorities said. Police believe Rodriguez delivered at Estevez' house.
Both girls were arrested on petit larceny charges. It was unclear whether either has a lawyer.
Rodriguez was taken to a hospital and remained there Friday, and a woman who answered the phone at a possible home number for her said she knew nothing about the matter. A message left at another possible number for Rodriguez wasn't immediately returned.
A woman who said she was Estevez's mother said she hadn't spoken with her daughter since her arrest and hung up.
The Associated Press is identifying both teens because in New York, older teens are treated as adults, and their cases are handled in criminal courts. Suspects between 7 and 15 may be charged as juveniles and their cases handled in family courts.
One of Rodriguez's neighbors, Zami Ford, told the Daily News she was unaware of the teen's pregnancy and stunned by the allegations.
"She's a good girl," Ford told the newspaper. "I can't believe she would do that."
Associated Press writer Jake Pearson contributed to this report.