If they can’t find a body they can’t prove it was technically murder. If they could do an autopsy they could then determine whether the child was still alive or had already passed when it as placed in the bag. They would also be able to tell if the baby had died due to “natural causes” (being too young to survive - lungs not developed, etc...), or if someone has hastened its death in some other way...
It seems they have charged this woman with everything the prosecution thinks it can prove. Again, with the evidence of the child’s actual body, murder charges would be VERY hard, if not impossible to prove (remember the whole “reasonable doubt” part of our system).
I will be praying they can somehow find this child’s body and can still determine cause of death when they do. The more time that passes though it is much less likely that they will EVER be able to prove what really happened... I will also pray that this woman is struck through her heart with guilt and if she did purposefully murder this child after he was born alive that she will be compelled to confess to her crimes... (I will also be praying for her daughter because I can’t even imagine the emotional and mental suffering she must be experiencing right now...).