And the only time conception is a non-arbitary point is if you don't think hard enough. For starters, to separate embryos from alive, unique induviduals that we consider in principle dead, then we need to introduce a truely arbitary thing caled potential for life.
Sure easy enough for those who end up born and living a long life, but that would for example mean that a foestus that will die before birth isn't a human so it's perfectly fine to abort it by the charming method DA described.
But we cannot have that so then you have run with that they might have a potential for life. Arbitary? No, of course not. :book:
And that is a legal hellhole if the law is going to follow that principle. For example, you cannot shovel up a bottle of liquor on child and force them to drink, so by the same principle a pregnant woman cannot drink. But it's also considered extreme negligence if a child aquire the liquor by accident, so a sexually active woman can't drink.
Kukri, the rhyme method have periods were the ovum get fertlized but not attached, aka no pregnancy.
http://jme.bmj.com/cgi/reprint/32/6/355.pdf