autopsy is not a magical or miraculous thing (spoilers)
First, excuse me my bad english.
I saw many people saying that they think that the ending of this movie is "unbelievable" because an autopsy would have been made in the dead child "revealing" the disease that the child had.
Autopsy is not a magical or miraculous thing that "reveals" everything that a person have. In fact, lots of diseases are not detected by autopsy.
Several criminal autopsy are inconclusive.
So even if an autopsy was performed maybe they didn't discovered the genetic disease that the child had. They weren't even searching for that, the mother had admitted a lethal injection.