The Washington County sheriff's office said Monday that it couldn't be determined if the infant son of 21-year-old Heather Snively died before or after he was removed from the womb.
Korena Elaine Roberts, 27, was to be arraigned Monday on a murder charge in Snively's death.
A medical examiner said Monday that Snively had died of "blunt force" and "sharp force" injuries.
Police said they found Snively's body Friday in a crawl space in Roberts' home in Beaverton, a Portland suburb.
Roberts told officers she'd recently given birth to the baby boy, said Sgt. David Thompson of the Washington County Sheriff's Office, and both were taken to a hospital. Doctors could not save the baby, and they alerted authorities after determining Roberts had not given birth.
Roberts, 27, was charged with murder in Snively's death and could face charges in the baby's death as well, Thompson said. She was being held without bond and the jail did not list an attorney.
Snively's mother, Heidi Kidd of St. Albans, W.Va., said Snively met Roberts a few weeks ago through Craigslist, the online classified service. Roberts told Snively she was pregnant and wanted baby clothes, Kidd said. They befriended one another and kept talking online.
"I guess they decided to meet and I guess that was the wrong thing to do," she said. "But Heather's always been a very trusting person."
Thompson said detectives have not confirmed the women met through Craigslist.
Roberts has two children, both under 10, who were staying with relatives and not at home when police believe Snively was killed, Thompson said.
Kidd said her daughter was eight months pregnant, and Snively's Facebook photo shows her lifting a pink shirt to expose a large belly.
Snively and her boyfriend, identified by Kidd as Christopher Popp, recently moved to Oregon from Maryland because he got a better job. Popp, the baby's father, reported Snively missing Friday night and is not a suspect, Thompson said.
Kidd said her first grandson was to be named John Steven.
"I'm still in shock; it hasn't hit me," she said. "I mean, that initial phone call; I just couldn't believe it. I just could not believe I was talking about my own child."
Investigators said Monday that Roberts had been telling friends and family members for months that she was pregnant, and told many people she would have twins.
They said Roberts may have been in contact with other pregnant women in the Portland area.
Snively had recently moved to Oregon from Maryland because her boyfriend and father of the child had found a job.