Size: True, 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.
Environment: 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.