Note that there's no evidence until after 5 months, this doesn't mean for sure that it isn't conscious before this. Lack of evidence isn't proof. It absolutely isn't self-aware, but its theoretically possible it can feel pain. However, it is impossible before third trimester because there is no cortex. And in the third trimester it's exceedingly unlikely. After birth it's still unlikely, but not 100% impossible.
But, in the case of severely disabled infants, yes, as Peter Singer argues. They are certainly not self-aware at that point, which means their life is replaceable. It's like killing a fish. You shouldn't cause it pain, but if you kill one and magically replace it with another fish, there's no effective difference. So, say if a mother has severely disabled infant, and this mother only can support one child, there's nothing wrong with 'replacing' this child with another one - killing the one that will live a life of pain and have a another healthy child.
But most parents would be horrified to kill their infant anyway, so it doesn't matter to much. I don't think many people are asking to legally kill their infant children.