John Frame writes:

In Jeremiah 1:5, Yahweh says that he knew the prophet before his conception, and appointed him as a prophet. So God knew that of all the marriages in Israel and all the various combinations of sperm and egg, one would produce a specific individual named Jeremiah equipped in advance to be a prophet. Many free human decisions led to the conception of Jeremiah in his mother’s womb, and God knew all those decisions in advance.
Frame, John M.. Systematic Theology: An Introduction to Christian Belief (p. 316). P&R Publishing. Kindle Edition.

This is not true. God does not claim to know Jeremiah before conception. God, instead, claims to “know” Jeremiah before Jeremiah is “formed” in the womb.

Jer 1:5  “Before I formed you in the womb I knew you, and before you were born I consecrated you; I appointed you a prophet to the nations.” 

The formation process, in the Hebrew mind, starts with unformed substance which then developes. John Calvin writes about Psalms 139:16:

16. Thine eyes beheld my shapelessness, etc. The embryo, when first   conceived in the womb, has no form; and David speaks of God’s having   known him when he was yet a shapeless mass, to kuema, as the Greeks   term it; for to embruon is the name given to the foetus from the time   of conception to birth inclusive. The argument is from the greater’ to   the less. If he was known to God before he had grown to certain   definite shape, much less could he now elude his observation.
Calvin, John. Calvin’s Complete Bible Commentaries (With Active Table of Contents in Biblical Order) (Kindle Locations 137956-137959). . Kindle Edition.

Jeremiah is continuing a tradition of prophetic calls from the womb. The idea is not an eternal calling or even a calling in some remote past. The idea is a calling from birth.