The formation of the life of a person in the womb is the work of God, and it is not merely a mechanical process but a work on the analogy of weaving or knitting: "Thou didst knit me together in my mother's womb" (psalm 139:13). The life of the unborn is the knitting of God, and what He is knitting is a human being in His own image, unlike any other creature in the universe... The destruction of conceived human life - whether embryonic, fetal, or viable - is an assault on the unique person-forming work of God.
John Piper