Laura Lynn Hickey, 25, was taken to a hospital in Centralia in Lewis Country Tuesday morning and told hospital staff she had had a miscarriage, KIRO-TV, Seattle, reported.
She told ER staff the fetus was still at her home, and they called police.
"They located the child in a container under a sink," Centralia Police Officer John Panco said. "Its head had been decapitated."
The Lewis County Coroner's office said a postmortem examination determined the infant was alive when it was born and its fatal injuries had been intentionally inflicted, Panco said.
Hickey was arrested Wednesday and booked pending the filing of first-degree murder charges, Panco said.