Scott Klusendorf has a great website on defending the pro-life message:
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Here's a great way to remember the only differences between a human embryos and me and you:
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 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 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 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-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, 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.
If you hear someone say that, "the choice to have an abortion is personal," you will soon realize how incoherent our culture has become. Because what this means is that they have rendered the question of whether or not to have their own child murdered to a "personal" matter. Who, on this earth, has a "personal right" to murder an obviously innocent human being?
Remember, the only legitimate questions to consider are: What is an abortion? What is murder? and What is a human being? All other questions or considerations are just rationalizations for murdering a human being for "its own good."