Better yet, when is it a human being?
The 64 Thousand Dollar Question. As is evident in this very thread, it depends on who you ask. SetAI will tell you it's not until the 3rd trimester. You probably believe it's at conception. Others might have other definitions or criteria. I personally believe that it's up to the parents to decide for themselves.
Aren't all human beings worthy of the right to live?
In most circumstances, yes. But nothing is ever black and white. What about massively dibilitating defects? Rape victims?