Bit of a derail into capital punishment arguments. She can't be sentenced on the supposition that she killed the child since this cannot be proved. There is no body, we do not know whether the child was born alive or dead.
She did have a psychiatric evaluation, she was tried and sentenced on the basis that she is sane, so the 'must be a mental illness' supposition is also misplaced. You can't extrapolate back from a terrible crime to say that the person who did it is mentally ill. Maybe, maybe not, but sane people are capable of gruesome things, as history and any glance at the headlines makes plain. This woman is very obviously troubled, mixed up, and odd, but as far as the court is concerned, not so much that she lacked agency or the ability to make different choices. She was sentenced accordingly.
People may find it comforting to 'other' these offenders as a distancing mechanism saying they must be ill but it is just, to me, a way of not facing up to the fact ordinary, even normal people sometimes do repellent things.