We have no issue applying this logic to drugs that would cause birth defects.

But legally no one cares if you kill it.

Yep. You nailed it

Defining a reasonable external agent perhaps?

For example a pharmacy can pierce your ear, but they can't look off your leg. So there's a big difference in degree of bodily autonomy even when it's all your own body.

When a child gets their autonomy they still aren't cogent. Even a young child needs help to stay alive. That places a demand on many other persons autonomy to assist them.

If we defined abortion on grounds of sensory faculties. Would killing a sleeping person be wrong? A blind deaf quadriplegic or severely affected leper would also not meet that criteria for 'resembling a person'

I stand with the vegan crowd and say defining killing is a dangerous game. I'd rather just not do it and rather that it was never done at all.