When speaking with someone about abortion and pro-life issues, what can you say? Many people simply will not accept our Christian reasoning; they reject the Bible as God's Word and will not hear the truth of our arguments. I ran across a simple acrostic a while back that has been helpful for me in questioning a pro-abortion stance. The acrostic is "sled," S.L.E.D. I wish I remembered where I found this but here it is nonetheless.
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 human 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 that relevant? Four year-old girls are less developed than fourteen year-old ones. Should older children have more rights than their younger siblings? Some people say that self-awareness makes one human. 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 why 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-human to human? If the unborn are not already human, merely changing their location can't make them valuable.
Degree of Dependency: If viability makes us human, than 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 because they share a common human nature.