Hm, I have always found this position hard to understand, which is why I am curious.

In fact, it is because of this 'thought experiment' that I define life to start at conception. To me, aborting a fetus a day before it is due is much much worse than doing so early on. A day before the due date, the fetus is completely viable, so to me there is little difference between this and murdering a newborn. We could think of the even more absurd scenario in which you kill the fetus once labor has actually started (after all, it can be a long process). What about two days before? Three days? This thought process always reminds me of Sorites Paradox

I agree that defining life to begin at some arbitrary point in the pregnancy makes very little sense, but I go in the opposite direction than you and say it begins with conception rather than birth.

Anyway, I don't mean to challenge your beliefs so don't take the above the wrong way or anything. Just explaining why I (and probably other pro-lifers) see conception as the logical starting point.