Why is the only difference between abortion and the unjustified killing of a child the short distance of the birth canal?
Stephen Schwarz uses the acronym "SLED" to show there are only four differences between a preborn child and a newborn: size, level of development, environment and degree of dependency. Although humans differ immensely with respect to talents, accomplishments, and degrees of development, they are equally valuable because they share a common human nature.
Yes, embryos are smaller than newborns and adults, but why is that relevant? Men are generally taller than women, but physical size doesn't equal value.
Level of Development
True, embryos and fetuses are less developed than you and I. Should older children have more rights than their younger siblings? Some people say that self-awareness makes one human. If that were true, newborns would not qualify as human beings.
Where you are has no bearing on who you are. Does your value change when you cross the street or roll over in bed? Location cannot change the essential nature of the preborn from non-human to human. If the preborn are not already human, merely changing their location, moving them eight inches down the birth canal, can't make them so.
Degree of Dependency
If viability makes us valuable human beings, then all who depend on insulin or kidney medication are not valuable. Conjoined twins who share blood type and bodily systems would also have no right to life.