@Shamar @noelle @Wolf480pl
there are some important distinctions to make here

1: life does not begin at birth, but neither does it begin at conception. life began a very long time ago and has stuck around since, spreading and changing but never yet stopping. whether you personally believe that it began as a single cell that became everything else or began as a fully formed pair of humans, that still holds true.

2. the thing that is significant about the birth of a child, then, is the coming into existence of a new person. that is, a sentient entity. a human is human for the entirety of its existence, but at any point it may or may not be a sentient, a person. the difficult bit here, of course, is defining how "intelligent and self aware" a thing needs to be before it can be considered sentient and a person. thankfully we have some societally decided-on precedents here. pigs, for example, are not considered to be people. since a just-born baby is less intelligent and self-aware than an adult pig, then, we can claim with certainty that it is also not yet a person. similarly, it is possible for a human to later in life experience some sort of brain death and again drop below these well-known and accepted boundaries, and thus cease to be a person while still alive.

given these conditions, the killing of a newborn child would be, although the killing of a human, not the killing of a person. since the widely-accepted moral stance on killing non-humans (like pigs) is "go ahead, it's fine", we can also conclude it would not be morally reprehensible (assuming an internally consistent system of morality, but that's something else entirely).

that all being said, personally i would not kill a newborn child or advocate doing so, for the same reason i would not kill a pig or advocate doing so: the line between sentient and non-sentient, person and non-person, is still and probably always will be "a very wuzzy line" (see https://www.ted.com/talks/jane_goodall_on_what_separates_us_from_the_apes/transcript ).

like "innocent until proven guilty", which aims to prevent the conviction of innocents in exchange for the aquittal of some guilty, walking near that line means the killing of a whole lot of non-people but potentially some people as well, which is not even a little bit ok at all, and people should really really stop killing each other please ;_;. it seems reasonable, then, to draw the line somewhere much further back (where it is currently, in pre-birth development). similarly i do not kill or eat animals i know to have any significant intelligence/self-awareness (like pigs or octopodes) and would advocate a line being drawn further back beneath their level as well