A fetus generally isn't believed to be a conscious entity (nor is a newborn for that matter). The important distinction vs. a kidney is more that it's something that likely will become conscious in the future.

The question of abortion is really interesting in that it scratches at a deeper moral question-- precisely why do we consider murder to be wrong?

In my view, there are a few different reasons. One is that we are preventing future consciousness for a being by ending the body's ability to be conscious. That one certainly applies to abortion. But another is that we hurt the people that have relationships with or dependencies on that conscious being. That one doesn't apply to abortion at all.