Yikes, the dreaded abortion question! As I understand it, it’s probably a combination of biblical verses, such as those in Exodus and in Genesis, which states that the soul of life is inspired by G-d into the body of the human at the moment of birth. Thus, at this moment when ensoulment takes place, the baby is a complete “person,” containing both body and soul. Moses613 can no doubt help out here, but I believe that the Orthodox not only permit but require abortion of the baby if the mother’s physical health is in imminent danger. On this point, all Orthodox rabbis agree. Beyond this, however, there are different rabbinical views based on Torah and Talmud teaching, including such issues as the psychological health of the mother who may have been the victim of rape or incest, for example, or instances when one unborn baby is literally jeopardizing the life of another, so that it is probable both will die if the less healthy baby is not aborted. Also, cases when the baby has a severe congenital disorder and will probably live for only a very brief time after birth in much pain.
The Reform Jewish view, as one might expect, allows more latitude concerning abortion, including rape and incest, congenital disorders, and other circumstances. Interestingly, however, Reform Judaism does NOT require abortion under any of these circumstances, including if the life and health of the mother are in jeopardy.