How so? Somewhere between conception and birth consciousness arises. Gametes are clearly not conscious as far as we can tell, and yet a baby is. So somewhere in the physical development that happens in between, consciousness arises as a result. The exact point is a topic of much debate.
Somewhere between mortal injury and decomposition, consciousness vanishes. Live humans, yet to succumb to a mortal wound, are clearly still conscious, yet a decomposed corpse is clearly not. So somewhere in between, consciousness ceases to be. We don't know yet exactly when, but I'd while say we've got it pretty narrowed down, the exact point currently a topic of debate in relation to organ donation.
This is all to say nothing of the how of consciousness, or what happens in between, but it's pretty clear there is a beginning and an end to it.
These are pretty basic observations that anyone can make. As for any presence of "consciousness" before or after these points, well, what evidence is there? None I've ever seen, just conjecture and speculation presented as fact.