The infant also died, but it could not be determined whether he was alive when he was taken from Heather Snively's womb, the Washington County sheriff's department said in a statement.
A 27-year-old woman, Korena Roberts, has been charged with murder in Snively's death. She appeared in court Monday but did not enter a plea.
A 911 call on Friday brought emergency workers to Roberts' house in the Portland suburb of Beaverton. Workers found blood on the floor and Roberts' boyfriend trying to revive the infant.
Investigators said Roberts claimed the baby was hers. At the emergency room, doctors, who were unable to revive the baby, determined that Roberts had not given birth.
Police said they returned to Roberts' home and found Snively's body in a crawl space. Roberts' boyfriend is cooperating and doesn't face charges, the sheriff's office said.
A medical examiner said Monday that Snively, who was in her last trimester of pregnancy, died of blunt and sharp force injuries.
"At this time, it has not been determined if she died because of head wounds she received or as a result of cutting injuries she received to her abdomen," the sheriff's office said.
A grand jury will hear the case, the prosecutor said, and Roberts might also face charges in the infant's death if lab tests determine he ever took a breath.
"The issue is was the child alive at all at any point in time," said Washington County District Attorney Bob Herman. "This is certainly unusual for its circumstances and nature."
Investigators said Monday that Roberts, who has two children younger than 10, had been telling friends and relatives for months that she was pregnant, and told many people she would have twins.
Snively had recently moved to Oregon from Maryland because her boyfriend and father of the child had found a better job.
Snively's mother, Heidi Kidd of St. Albans, W.Va., said Snively met Roberts a few weeks ago through Craigslist, the online classified service. Police said Monday they are still trying to confirm that.
Roberts told Snively she was pregnant and wanted baby clothes, Kidd said. They befriended one another and kept talking online.
Kidd told The Associated Press on Sunday that 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."
Neighbors on Monday said Roberts bought a stroller at a garage sale and set up a crib on the front lawn of her rental home. They said Snively was seen at Roberts' house more than once.
On Monday outside the house, there was a blue plastic play pool, a broken yellow play tractor, offerings of flowers and a handwritten note, saying, "May God be with you."