I was surprised that I hadn’t really thought this through before: we clearly do things to babies that we would not tolerate doing to adults without their permission (e.g. take blood, deny them property rights). I understand that newborns would find it hard to give permission just as we would find it hard to understand their consent.
This suggests that there is a continuum of availability / applicability of rights, even though the rights themselves feel pretty black and white in most cases. This continuum effect subtly changes the argument around rights from conception to death. The categorical imperative sings a siren song here.