According to the Buddha's teaching, the self is a phenomenon that arises when several causes, or aggregates, come together, namely form, sensation, perception, "impulse" (samskara), and consciousness. Without all five causes, there is no self and there is no person.
A fertilized ovum has form, and it may even have sensation (feeling), but I doubt that many people believe it also has perception (awareness of the feeling) and consciousness. At some later point in development, the fetus undoubtedly does, and can properly be called a "self" or a "person."
Applying this to the abortion controversy, it could be said that terminating a pregnancy in it's earliest stages is not "murder," as there's no "person" yet there. There may still be "killing," as one is rendering living tissue non-living, but in the absence of all five aggregates, it's not a "person" that's being killed but just some of the underlying aggregates before a person is formed.
One needs to be careful here, though: by the same argument, if consciousness is absent, then there is no "self" or "person" either, even if all the other aggregates are present, but I doubt many Buddhists would not consider it to be murder to kill an unconscious person.
However, as pointed out earlier, in Buddhism it's left to the individual how to observe the precepts and how to interpret the dharma.