Following interviews with UPMC medical staff, Nelson and her husband, Shawn, police now believe Nelson suffocated her son several times from August through October, including at least two times at the UPMC Children's Hospital.
Police contacted Child Protection Services in Pennsylvania and West Virginia, who immediately took custody of the child.
Nelson was taken to the University of Pittsburgh Medical Center's Western Psychiatric Institute and Clinic for evaluation. She has not been arrested yet.
"She is in a hospital in Pittsburgh now and hasn't been picked up on the charges," Miller said.
Miller said as far as he knows, the infant is Nelson's only child. The boy is in stable condition at Children's Hospital's Intensive Care Unit, according to the press release.
Miller said he "would not rule out" charging Nelson in West Virginia.
"You've got a 4-month-old little boy. They can't be subjected to that kind of stuff. We're not going to put up with that," he said. "Once we get all our information together, we'll send it to the prosecutor."