It is generally accepted that some lives are more valuable than others. If we did not accept that, we would not kill living things and eat them. Because of that principle, a mother's life is more valuable than that of the child, and it is morally acceptable to abort a pregnancy if her life is in danger. The mother is fully self-conscious and rational. Furthermore, she has an established life. That means she has a past and a future that she is aware of, and she has relationships with other people. She has a job, friends, family that she knows and has relationships with. She contributes to and has an established place in society. A fetus is not fully self-aware or rational, by many, that alone makes it less valuable than the mother's life. Furthermore, it is not aware of a past or future, and it does not have an established place in society or extensive relationships. Because of that, it would be worse to lose the life of the mother than that of the fetus. Therefore, if the mother's life is in danger, it is morally permissible to abort the fetus.
With the advances of modern medicine, doctors have sufficient knowledge to save both the mother and child. It is rarely truly necessary to abort the fetus to save the life of the mother. Because of that, it is wrong to take action to kill the fetus to supposedly save the mother. 
[P1] An adult is self-conscious. [P2] A fetus or preborn is not fully self-aware. [P3] The loss of a self-conscious life is worse than that of one that is not.