You nailed the question, John.  Unfortunately, not the answer.  The acorn and oak tree do not differ in essence any more than a new-born differs in essence from an adult, or a new-born from a newly conceived fetus.   Is there a difference between a new-born and an adult?  Of course, but not a difference in essence.  For which reason you may not kill either an infant or an adult.  The same holds for the relationship between a new-born and a newly conceived fetus.  Since they do not differ in essence --- since each is human in essence --- you can no more kill a newly conceived fetus than a new-born child.   

You may now hit the laugh key, DunceInKansas.