to save a mother's life.
According to the Mishnah (Sanhedrin 4:5):
Whoever destroys one life is as if he destroyed a whole world, and whoever preserves a life is as if he preserved the whole world.
Apart from an overall regard for the sanctity of life, Judaism finds other reasons to forbid abortion:
killing a fetus breaks God's command to populate the world
killing a fetus destroys something made in God's image
killing a fetus is wanton destruction of part of God's creation
killing a fetus destroys something that could become a being
killing a fetus is an unjustifiable act of wounding
https://www.bbc.co.uk/religion/religions/judaism/jewishethics/abortion_1.shtml