I think we are not fully born until the adoption at the resurrection of the body.
For we know that the whole creation groaneth and travaileth in pain together until now. And not only [they], but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, [to wit], the redemption of our body. For we are saved by hope: but hope that is seen is not hope: for what a man seeth, why doth he yet hope for? But if we hope for that we see not, [then] do we with patience wait for [it]. Romans 8:22-25
The word born means procreate or conceive.
G1080 γεννάω gennao (ǰen-naō') v.
1. (properly, of the father) to procreate.
2. (by extension, of the mother) to conceive.
3. (figuratively) to regenerate.
So a fetus has been conceived, but isn't fully born for 9 months. Something born in the womb can die. The born cannot be unborn idea has shades of gnosticism. They believed once they received gnosis they could never perish no matter what they did because they were spiritual beings that could never die