Brainstorm

[Catholic_Lady]

At the moment of conception it is, biologically, a human entity. I don't know of any other fair way to define that point other than by what it actually is. It would just be arbitrary otherwise, and when it comes to defining another being's humanity, history has shown us that being arbitrary on this can lead to horrific things. (think holocaust, slavery, the slaughtering of indians in the new world, etc... all in the name that these people weren't actually "human")

But why at the moment of conception? Do we consider a sperm quasi-human, prior to this?

I think we should consider whether it can suffer and maybe think(or be self-aware) in any way before we actually define it as a person and consider it with the same dignity we consider other humans. Otherwise we are merely talking about its potential to become a human being.

Because at the moment of conception a brand new set of human DNA is formed. A sperm is just a sex cell from the man's body, it isn't its own separate entity. 

To address your last paragraph though, a human fetus begins to feel pain about midway through the second trimester... at least as far as we can tell, though it could be before that. But I don't think the ability to feel pain should be the indicator to being human. If you are comprised of human DNA, you are a biological human being, whether you have the ability to feel pain or not, imho.