It’s actually a pretty simple question, and has a simple, straightforward answer. The fetus does not become alive until its survival needs can be feasibly met by someone or something other than the mother. Until it is biologically capable of surviving the death of the mother, it is alive only as a part of the mother’s body.
An infant does require considerable support. It will die if neglected. But, the support an infant requires can be provided by any caregiver. Dad, grandma, or an older sibling can feed an infant. Doctors can provide it with IV nutrition.