The mother's sister called 911 Tuesday stating she needed an ambulance right away to their Elma home for a woman needing urgent medical attention after a possible miscarriage.
"She didn't even know she was pregnant," the sister told dispatchers, who replied: "OK, so how do you think it's a miscarriage if she didn't know she was pregnant?"
Medics arrived and took the mother to an Olympia hospital.
But once the woman got there, investigators say doctors realized they weren't getting the whole story.
"When she got to the hospital, the doctor found evidence that clearly spoke to her having given birth," said Grays Harbor County Sheriff Rick Scott.
That's when deputies went back to the home and discovered the body of a newborn girl inside.
"It's a suspicious death," Scott said.
It's still undetermined if the child was born alive, or stillborn.
When detectives confronted the mother about giving birth, but reporting a miscarriage, they say she told them the child was stillborn and she was afraid to report it to police. The sheriff says she has custody issues with two young sons who no longer live with her, and they're seeing if that played a role.
"It may have somehow factored into the decision she made to not be forthcoming with exactly what happened," Scott said.
An autopsy on the newborn is scheduled for Thursday to determine if the baby was alive at birth. In the meantime, the mother is not being held by police.