Does the Constitution specify what is a human, at what stage, etc?
Oh wait, it DID.
A human - according to the Constitution - is as has been understood by their English forefathers, and that is, an adult Protestant British or Teutonic upper class land owning man of means.
Women, like slaves and also as children still today (including those unborn uterine 'cysts' women carry for 9 months prior to the stork bringing home baby) were chattle, and not bestowed any rights other than as property of the male head of the household.
Rights, obviously, as exemplified by their flexibility, are social constructs. Pretzelating semantics does not define or establish who they are granted to, upper class men decide if the lowly peons beneath them should be granted the worthiness to allow the extension of whatever 'rights' towards them.
If an unborn baby wasn't human or alive or not a person, then there'd be no need to abort it.
Abortion, btw, is an act that said unborn baby did not consent to, regardless if it is for (primarily, with certain rare exceptions) the convenience of the mother, and a violation of her biological imperative and primary function as a reproductive organism. If she believes she is not capable of being a mother, then she should opt for sterilization. And yes, this is not including cases of rape/incest and risks to her health. Same for the fathers. Heck, sterilize the masses and grant the 'right' to breed to only the less genetically weak. Humans are de-evolving at a rapid clip and most make awful parents. The time to do something about it was long ago.