Original sin in essence has to do with original "insanity."
The reason that a child can have original sin before they have even experienced life outside the mother's womb is that the baby's brain is like a computer that has yet to obtain an effective operating system. So, until that child is able to understand the concept of "pure wisdom" or knowledge that is without reactive emotional pollution such as greed, ego, anger, hate, jealousy, sadness, frustration, etc. then they are considered to be insane or "with sin." The brain can not operate effectively while experiencing such reactive emotions.
Technically speaking, sin means "to miss the mark" like an archer's arrow missing the target or being off-center i.e. incomplete or not "yet" perfect.