Police said late Sunday they found Heather Snively's body on Friday in a crawl space in Korena Elaine Roberts' home in a Portland suburb.
They had been called there because a newborn wasn't breathing. The baby later died.
Roberts told officers she'd recently given birth to the baby boy, said Sergeant David Thompson, 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 in custody.
Snively's mother, Heidi Kidd, 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.
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."
AP