MORAL STATUS
A living things is said to have moral status if there are moral restrictions on killing it or using it for our own purposes which are grounded in the nature of the creature. In other words, if it is wrong to use or kill the creature because of what it is, then it has moral status. Different organisms may have different levels or degrees of moral status. They might have:
No moral status: There are no restrictions on killing or using it (such as grass, insects or bacteria).
Partial moral status: There are some restrictions on killing or using it, (Such as Elephants, Dolphins, Gorilla's)
Full moral status: there are many stringent conditions on killing or using it (such as include you and other people).
THE 'FUTURE LIKE OURS' ARGUMENT
One of the reasons it would be gravely wrong to kill a person like you is that doing so would deprive you of a very valuable future.
Killing an embryo or foetus deprives it of a future like yours, one with similar human goods and experiences.
Therefore, it would be gravely wrong to kill an embryo or foetus.
THE 'HUMAN DIGNITY' ARGUMENT
The right to life, or dignity, is the most fundamental right a creature can have. If a creature has it, it must be by virtue of what the creature most fundamentally is.
We all agree that you have a right to life.
What you most essentially are is an individual human being.
'Embryo' and 'foetus' are stages in the life of an individual human being.
Therefore the human embryo or foetus has dignity, or a right to life.
SENTIENCE AS A CONDITION OF MORAL STATUS
Moral status is about protecting a creature's interests.
Interests are things a creature can take an interest in.
Creatures need sentience to take an interest in something.
A necessary condition of having moral status then, is the presence of sentience, of being aware of things.
Applied to early human life: the foetus has no moral status until it has the capacity for sentience or consciousness, which is about half way through gestation.
THE GRADUAL DEVELOPMENT OF MORAL STATUS
A newly fertilized egg or an embryo is a partial realization of a fully-fledged human being.
According to gradualism, just as early human life develops in to a fully human being, slowly, step-by-step, early human life's moral status develops in to full moral status slowly, step-by-step, over the course of pregnancy.
SUMMARY:
A FUTURE LIKE OURS: early human life has full moral status, because embryos and foetuses have a future full of human good.
HUMAN DIGNITY: Humans in the early stages of development have full moral status simply because they are human beings.
SENTIENCE: early human life has partial moral status because only conscious beings have any moral status and embryos and early foetuses are not conscious.
GRADUALISM: early human life has partial moral status because moral status grows gradually along with foetal development.
THREE PERMISSIVE VIEWS OF ABORTION:
THE STRONGEST POSITION: Abortion up through the mid-point or so of pregnancy is always morally permissible because early human life has no moral status.
THE MODERATE POSITION: Even early in pregnancy, the embryo or foetus has partial moral status. Given this value, abortion is only morally permissible if continuing the pregnancy will significantly diminish the woman's well being (or for any reason, the pregnancy did not result from voluntary actions).
THE DOMINION POSITION: Even if abortion is the destruction of something with partial moral status, aborting is always morally permissible because of the dominion rights women have over their bodies and/or reproductive lives.
DEONTOLOGICAL APPROACH: Kant's categorical imperative would say not killing is a universalizable moral action.
Sources:
Margaret Little, Director, Kennedy institute of Ethics.