There are a couple of rabbit trails worth tracing here. Primarily because Jeremiah 1 is used for modern-day issues.
Abortion/Pro-Life
This same verse is also used to show the wrong of abortion. That God has plans and purposes in the womb. And for this, I would say that this is valid. Because if you look throughout Scripture, God is seen as doing a work and calling people while they are in the womb.
Psalm 139:13
For you created my inmost being; you knit me together in my mother’s womb.
Luke 1:41 When Elizabeth heard Mary’s greeting, the baby leaped in her womb, and Elizabeth was filled with the Holy Spirit.
Galatians 1:15 But when God, who set me apart from my mother’s womb and called me by his grace, was pleased
When I read these and other verses, it is clear that God sets apart purpose while He is knitting someone together in their womb. The womb is clearly the place of God’s workmanship, and so yes, while this verse isn’t about abortion directly, it indirectly teaches a truth that is applicable here to the question of life, purpose, and God’s workmanship.
The reason babies have always been targeted by satan is that when he destroys a life, the purposes of God in that life are destroyed. God can still accomplish his purposes through different people and in different ways, but His original has been destroyed.
Pre-Existence?
Some Latter Day Saints (Mormons) use Jeremiah 1 to prove that there was a pre-existence of souls. This is based on their belief that god was a man like we were, but achieved glory through his good work. Then he had sex with Mary and produced many spirit children. These spirit children were then sent to earth, received physical bodies, and are here to learn right and wrong. They need to have many children so that the pre-existent spirits can come to good homes. (This video does a good job of explaining their belief – Here).
So basically they say that all humanity had a pre-existence, and we too will one day become gods of our planets, with men having eternal sex and multiple women and women being eternally pregnant, sharing their husbands.
One of the Scriptures that some LDS use to support this view is here in Jeremiah 1:5.
“Before I formed you in the womb I knew[a] you,
before you were born I set you apart;
I appointed you as a prophet to the nations.”
Is Jeremiah teaching a pre-existence here? The Hebrew word for “knew” is also the word for “chose.” Nor is there anything for us to believe that there were spirits or that Jeremiah knew God previously. And the whole idea of there being millions of other gods does not coincide with Scriptures like Isaiah 43:10.
Furthermore considering there are no other Scriptures that would indicate a pre-existence, and the fact that this just as easily could mean he had purpose prior and planning prior to forming Jeremiah in the womb, it would be more likely that this is the latter.
Plans and purpose always precedes creation. This does not necessitate a pre-existence, but rather the purposes of God were already in his heart when God was forming Jeremiah in the womb.