If the child would die without the parent you would. Maybe not for petty crimes, but if the parent is a murderer they need to be in prison. If they are in prison, the fetus needs to be in prison to escape death.
No, we don't do that. Most children would die without some kind of care, but we don't put them in prison if their parents are criminals. We make sure they are cared for by someone else.
(The systems for that are in terrible shape too, but that's a separate issue.)
(The systems for that are in terrible shape too, but that's a separate issue.)