"Thine eyes did see my substance, yet being unperfect; and in thy book all my members were written, which in continuance were fashioned, when as yet there was none of them" (Psalm 139:16).
This is an amazing verse, testifying as it does to the omniscient fore-planning of our Creator for each human being. Each person has been separately planned by God before he or she was ever conceived; His eyes oversaw our "unperfect [not imperfect, but unfinished] substance" -- that is, literally, our embryo -- throughout its entire development. Not only all its "members," but also all its "days" (the literal implication of "in continuance") had been "written" in God's book long ago.
While modern evolutionists argue that a "fetus" is not yet a real person, and so may be casually aborted if the mother so chooses, both the Bible and science show that a growing child in the womb is a true human being. Instruments called fetoscopes have been able to trace every stage of embryonic development, showing that each is distinctively human, never passing through any non-human evolutionary stages, such as the evolutionists' theory of "recapitulation" would imply.
Not much is known about how a baby receives its soul, but the baby is surely an eternal human being from the moment of conception, with all its future days already well known in the mind of God, "when as yet there was none of them," as our text points out.
But that is not all. All those who are saved (or, like the innocents who die before birth, "safe" in Christ) and whose names, therefore, are "written in the book of life of the Lamb slain from the foundation of the world" (Revelation 13:8), are also predestined "to be conformed to the image of His Son" in the ages to come (Romans 8:29). HMM