What does the Bible say about abortion? Is a child's life worth less than an adult's? Does life begin at birth or conception?
Judaism contains a lot more wisdom than what you can read in the Bible. There's a very rich tradition that tells us how to understand the Bible and how to apply it. That tradition has been passed down from teacher to student in an unbroken chain since Moses. Eventually, much of it was written in the Mishnah and the Talmud, along with many of the discussions and later enactments that were based on these traditional teachings.
For example, the Bible tells us "Thou shalt not kill." But what does that mean? What if someone is going about killing others? What if he is trying to kill me?
So our tradition tells us that the Bible is not talking about those cases. If someone is out to kill you or other innocent people, you need to protect those innocent lives, even if it means killing the murderer.
This applies to an abortion, as well. A fetus is a potential life, so we are not allowed to kill a fetus. However, if the fetus is endangering the mother's life and the only way to protect the mother is by taking the life of the fetus, then we must do so.
However, this is all only as long as the fetus is a life-in-potential. Once the baby's head has emerged from the birth canal, s/he has become a full-fledged human being of the same status as the mother. Even though the mother has a family to take care of and has proven herself viable and valuable, we consider this a matter of one life versus another. At that point, we can't give precedent to either life. Life, according to our tradition, is not something to which you can apply relative values.
There is a very large literature on this topic. And as in all areas of complex halachah (Jewish law), every case needs to be individually evaluated by a rabbi -- who when necessary will consult with medical professionals and/or rabbis who are experts in this particular field of law.