Although Embryo offers an argument against killing embryos as part of scientific research, many of its points can be applied to the issue of abortion. It is written by Robert P. George, a professor of jurisprudence at Princeton University, the same university where the odious Peter Singer is employed.
His arguments go something like this--I hope I don't butcher them:
1. Ontologically, we are human animals.
2. The merging of male and female gametes creates a new human organism, a new member of the species Homo sapiens.
3. There is a transformation in kind--in category--as we move from gametes to embryo. An embryo is a new human organism--a gamete is not. An embryo is thus an ontologically different kind of entity than a gamete, but there is no change in kind for the rest of this embryo's life. There are different stages of maturation--fetus, infant, child, adolescent, adult--but there is a scientific consensus that there is no change in the kind of organism being discussed. It is a human organism and no other kind of organism at all stages. How far back do "I" go? To the moment of fertilization. All other points are arbitrary.
4. Embyros are categorically different from somatic cells. They are not part of a human organism; they are a human organism. If a somatic cell were turned into an embryo, it would be a human organism.
5. What gives an embryo its humanness is its nature. Its nature is programmed into it from the moment of its existence. It does not become human when it develops sentience, or the immediately exercisable ability to think, or when it develops self-awareness. These are all arbitrary points (not to mention that they imply moral status for sentient beings all of species, or they remove moral status for babies who have not developed the mental abilities yet). The nature that leads to self-awareness, for example, is in the embryo in the same way that it is in the newborn.
6. People who privilege sentience, self-awareness, etc., are mind-body dualists. They say, "I am a mind who possesses a body, and I do not exist unless my mind exists." Others will argue, "I am my brain." That is brain-body dualism. The truth is that humans are animals. The correct statement is, "I am a body--an integrated system." We are integrated organisms--not minds or brains. When I look in the mirror, I don't see a mind. I don't see a brain. I see a naked ape with glasses.
7. Since there is nothing but non-essential differences between embryos and more mature humans, they deserve moral status like mature humans. They thus certainly deserve the most obvious of all human rights--the right not to be killed. These embryos are humble: they're more than willing to dispense with the right to a living wage.
UPDATE: George does not make this point, but I would also argue that psychological biases lead people to view an embryo as less than human. First, since we can't see it, we reduce it to something less than a human organism. Second, we have a bias which says, "If it looks like a human, it is one; if it doesn't, it isn't." This bias enables people, for example, to have sex with plastic dolls or to think the female alien in Avatar is hot.
In college, I worked nights as a security guard and occasionally felt certain that there was some evil presence in the dark, empty building I watched. The reasonable thing for me to do was to recognize and ignore the bias. It would not have been reasonable to call the Ghostbusters.