Do all human beings have an equal right to life or do humans come to be at one point, but only become valuable later in virtue of some acquired characteristic?
Philosophically, there is no morally significant difference between the embryo you once were and the adult you are today. As Stephen Schwarz points out using the acronym SLED, differences of size, level of development, environment, and degree of dependency are not relevant in the way that abortion advocates need them to be.
Yes, embryos are smaller than newborns and adults, but why is that relevant? Do we really want to say that large people are more valuable than small ones? Men are generally larger than women, but that doesn't mean that they deserve more rights. Size doesn't equal value.
Level of development:
True, embryos and fetuses are less developed than you and I. But again, why is this relevant? Four-year-old girls are less developed than 14-year-old ones. Should older children have more rights than their younger siblings? Some people say that self-awareness makes one valuable. But if that is true, newborns do not qualify as valuable human beings. Six-week-old infants lack the immediate capacity for performing human mental functions, as do the reversibly comatose, the sleeping, and those with Alzheimer's Disease.
Where you are, has no bearing on who you are. Does your value change when you cross the street or roll over in bed? If not, how can a journey of eight inches down the birth-canal suddenly change the essential nature of the unborn from non-valuable tissue mass to valuable human being? If the unborn are not already human and valuable, merely changing their location can't make them so.
Degree of Dependency:
If viability bestows human value, then all those who depend on insulin or kidney medication are not valuable and we may kill them. Conjoined twins who share blood type and bodily systems also have no right to life.
In short, it's far more reasonable to argue that although humans differ immensely with respect to talents, accomplishments, and degrees of development, they are nonetheless equal (and valuable) because they share a common human nature. Humans have value simply because of the kind of thing they are, not because of some acquired property they may gain or lose during their lifetimes.
Abraham Lincoln raised a similar point with slavery, noting that any argument used to disqualify blacks as subjects of rights works equally well to disqualify many whites.
"You say ‘A' is white and ‘B' is black. It is color, then: the lighter having the right to enslave the darker? Take care. By this rule, you are a slave to the first man you meet with a fairer skin than your own.
"You do not mean color exactly—You mean the whites are intellectually the superiors of the blacks, and therefore have the right to enslave them? Take care again: By this rule you are to be a slave to the first man you meet with an intellect superior to your own.
"But you say it is a question of interest, and, if you can make it your interest, you have the right to enslave another. Very well. And if he can make it his interest, he has the right to enslave you."
If humans have value only because of some acquired property like skin color or self-consciousness and not in virtue of the kind of thing they are, then it follows that since these acquired properties come in varying degrees, basic human rights come in varying degrees. Do we really want to say that those with more self-consciousness are more human (and valuable) than those with less? As Lee and George point out, this relegates the proposition that all men are created equal to the ash heap of history.
Sadly, opponents of the pro-life view believe that human beings that are in the wrong location or have the wrong level of development do not deserve the protection of law. They assert, without justification, the belief that strong and independent people deserve the protection of law while small and dependent people do not. This view is elitist and exclusive. It violates the principle that once made political liberalism great: a basic commitment to protect the most vulnerable members of the human community.
We can do better than that. In the past, we used to discriminate based on skin color and gender, but now, with elective abortion, we discriminate on the basis of size, level of development, location, and degree of dependency. We've simply exchanged one form of bigotry for another.
In sharp contrast, the position I have defended is that no human being, regardless of size, level of development, race, gender, or place of residence, should be excluded from the moral community of human persons. In other words, the pro-life view of humanity is inclusive, indeed wide open, to all, especially those that are small, vulnerable and defenseless.