[Gamblor]

It's quite simple:

An unborn baby is not entitled to life because it is not aware of its own life. Self-awareness is the defining human characteristic.

Human life begins at birth, not conception.

The obvious counter argument would question the respect of animals despite non-self-awareness.

Animals are different, because they sustain the world that provides life for humans.

Thus, we must respect animals/plants/etc, because without them, humans would not continue to survive - it only follows that while animals may not be sentient and self-aware, they nonetheless require human respect for their roles in the ecosphere.

Ultimately, this argument only proves that humans are nothing more than self-aware animals. It's the same reason sex feels good, we find babies cute, and we care for our elderly:

It's all biological imperative. Sex is good because we need incentive to pro-create. Babies are cute because we're hard-wired to perpetuate the species, and it's easier to care for cute things than ugly things. We care for our elderly for what they can provide and what they have already provided: wisdom and experience that makes our own lives easier. And babies who are unborn are not yet babies, and since they aren't cute, only the Christian religious camp, which has never really let logic define itself, has any reason to demand their life.