This is complete nonsense. As far subjectivity, everything is subjective. Objectivity is, in total honesty a myth. I absolutely believe in a concrete, material universe, but we can't escape our brains, until we can get outside the brain (Which is impossible, as this relies on Cartesian dualism which has proven to be bogus.) With that out of the way, this is in no sense arbitrary. We are brains, you are a brain, the rest is just packaging. All we are, the entity you know as 'I' is comprised entirely out of a couple pounds of gray matter. Brain development is a scientific, measurable, rational criteria as a prerequisite for personhood.
You only have to ease your conscience if you've done something wrong. The wrong you're charging is determinent on the embryo's qualification as a human being, which, in the overwhelming majority of cases, it doesn't even come close.
Again, any attempt at a secular argument against abortion is an exercise in futility. This is a religious issue. It's irrational, but that's religion all over. However, none of us should be required to adhere to religious prohibitions, if we do not share those religious convictions.