In what way can a cluster of cells have rights, why should we even care?
By all means, the thing we care about in a human is the human mind. It would make no sense to care about the rights of a bunch of unfeeling, mindless organs. It doesn't really make sense to talk about a liver's right to life, we do not care about the "consent" of a heart muscle. Even if we start attaching one to another it doesn't really make any sense. Up until we add a mind.
Now, importantly, we can also separate this in the other direction. If we were to take a human mind out of a human body, we would still most likely think of that new being as having the same rights as the original. Therefore, the meat sack really is irrelevant.
Now we come to another issue. The "potential" or "inevitable" fully formed person. Let us consider our meat mannequin again. If I start building a person out of whole, living organs, but do not give it a functioning brain, is it a person? By the previous logic, no. If we put a brain and a mind into it, it is a person.
Now what happens if we have the "potential" or "inevitable" personhood? I, as the mad scientist, am about to put a functioning brain into my mindless organ sack and throw the switch. Suddenly, my lab partner decides to stab the organ sack until it dies. Has my lab partner committed murder? Not any more than he could murder any of the organs separately. The body is destroyed, but no person is.
By all means, then, I do not recognize a cluster of cells, even if it has a unique DNA blend. even if it has the "potential" to become a fully formed person, to be a person. It therefore makes total sense to say that abortion before the development of a mind can happen is not murder.
Now, weeing as we know that the brain is in fact the organ that deals with the mind directly, we can easily recognize that before the brain begins formation, it makes no sense to talk about the embryo being a person before the brain begins development.
What comes after that point is obviously more murky, but I can not see any reason why aborting a two-week old embryo is a bad thing.