Roberts, who was arrested and charged with murder, will be arraigned later today, said Thompson. She is currently being held without bail and a lawyer for her was not immediately known. Thompson said that Roberts could face additional charges of murder regarding the unborn baby.
Thompson declined to say if Roberts made any statements at the time of her arrest or during the investigation.
Snively and her boyfriend, Christopher Popp, recently moved to Oregon from Maryland because of a job opportunity. Popp, the father of the child, reported Snively missing, according to Thompson.
While the cause of the baby's death is still not known, Snively's death is eerily similar to past crimes involving pregnant women.
In July 2008, Phiengchai Sisouvanh Synhavong was arrested for allegedly binding together a pregnant woman's hands and feet, removing her baby and stabbing the woman to death. Police in Seattle said that Synhavong claimed that the baby was her own.
Earlier that year, two other women in Illinois and Missouri were convicted of similar crimes.
Lisa Montgomery of Melvem, Kan., was sentenced to death in April 2008 after killing Bobbie Jo Stinnett, a woman she met in an online chatroom. Eventually meeting her victim in her Missouri home and cutting Stinnett's baby, then in the eighth month of gestation, from her womb. Police said Montgomery used a kitchen knife to cut the infant from its mother's stomach.
Twenty-six year old Tiffany Hall committed a similar crime, killing her pregnant friend and cutting her unborn child from her womb in a bathtub.
Dr. Michael Welner, a forensic psychiatrist and head of The Forensic Panel, told ABCNews.com that these types of crimes are generally acts of "desperation."
"Characteristically, in the past, cases come up where women kill to establish their fertility," said Welner.
"It's a crime more common in primitive cultures, where women feel the need to establish their mothering identity," he said. "They can't get away with this in hospitals here. They check too thoroughly."
ABC News' Dean Praetorius contributed to this report.