The Maverick explains:
It is often said that a human fetus is a potential human life. Not so! A human fetus is an actual human life. Consider a third-trimester human fetus, alive and well, developing in the normal way in the mother. It is potentially many things: a neonate, a two-year-old, a speaker of some language, an adolescent, an adult, a corpse. And let's be clear that a potential X is not an X. A potential oak tree is not an oak tree. A potential neonate is not a neonate. A potential speaker of Turkish is not a Turkish speaker. But an acorn, though only potentially an oak tree, is an actual acorn, not a potential acorn. And its potentialities are actually possessed by it, not potentially possessed by it. The typical human fetus is an actual, living, human biological individual that actually possesses various potentialities. So if you accept that there is a general, albeit not exceptionless, prohibition against the taking of innocent human life, then you need to explain why you think a third-trimester fetus does not fall under this prohibition. You need to find a morally relevant difference -- not just any old difference, but a difference that makes a moral difference -- between the fetus and any born human individual.