A pillow placed over an infants face produces the same injuries as running it over with a lorry? Either that is one soft lorry or you mistook the tyre for a pillow.
1) Maybe this is the case, but what reason is there to assume that the mother is the culprit?
2) Does a detailed CSI-level investigation take place when an infant dies of an apparent SIDS death? I'm sure the parents are delicately asked about what happened for somebody to fill out the paperwork, but if there's no obvious bruising or broken bones or anything, how intense is an actual investigation?
2) Occam’s razor. I will give you that sleeping with newborns in bed and accidentally smothering them happens all too often, but anything more malicious can in my opinion be safely discarded, because parent’s killing their child require way more presupposition and proof would be on your side.
If a heavy, dense pillow is placed over an infants face without applying force I am sure it could smother the baby without leaving any injury. Of course an autopsy will reveal asphyxiation but no injury could leave the door wide open for an accidental death ruling.
Except that there is exactly zero evidence to support this.
Those sort of injuries would be as readily apparent as if you'd run the child over with a lorry, and they are absent.