Let me be clear, this answer is for Christians. If you don't believe in Christ, the points might not be meaningful to you.
We could argue about the origin of that shared belief, but most people accept it.
This is not THAT advocacy. This is based on Christianity.
There are a couple things I'm not going to discuss here. One, is detailed law from Deuteronomy and Leviticus.
The second are verses about being "knit together" in the mother's womb, and the like.
I'm going to offer a little different spin.
God did this because he loves us.
At the very beginning, in Genesis, God creates men and women as a special creation, separate from the animals.
We are special to him. Human life has value because he values us.
You know, Christian, how that worked out.
Adam failed, and so have we.
Does the Bible report that the Messiah descended from Heaven to save us?
Jesus was a zygote. Jesus was an embryo. Jesus was a fetus. Jesus was a baby.
Elizabeth's child, John the Baptist, leapt for joy in Elizabeth's womb at the presence of the developing Jesus in Mary's womb.
Just like us, Jesus was a unique, valuable individual from his very beginning.
So, while the Bible does not mention abortion specifically, the entire point of the Bible for a Christian - God's plan of salvation because of his love for us, argues against abortion.
The Bible says nothing about abortion while saying everything about it. /end