Do you think just ANY woman would be chosen to be the mother of God? She was born without original sin. An angel came to her (this IS in the bible) and told her she would conceive a son by the Holy Spirit.
She said yes, but can you imagine how scary that was for her? It would be scary enough now, but she could have been stoned to death if Joseph walked away from her.
By the way, we don’t make stuff up. Many many miracles have been attributed to the Blessed Mother.