Abortion is of overriding concern because it negates two of our most fundamental moral imperatives: respect for innocent life, and preferential concern for the weak and defenseless.
US Bishop's Resolution on Abortion (1989)
The Church teaches that every human being has the rights of a person from the moment of conception.  Every innocent person has an "inalienable right to life."  The killing of an innocent human being is "always morally evil and can never be licit either as an end in itself or as a means to a good end."  For this reason formal cooperation in abortion is always a grave crime against human life. 
For many women the decision to have an abortion is difficult and painful. Some mothers may obtain an abortion in order to protect "certain important values" such as her health or the living standard of her family. Some even believe that the the child's living conditions would be such that it would be better for the child not to be born. "Nevertheless, these reasons and others like them, however serious and tragic, can never justify the deliberate killing of an innocent human being." 
What about cases of rape or incest?
Rape is an intrinsically evil act. It is a crime against justice and charity. Rape of a child by a parent is graver still.  Nonetheless, a child that is the result of such a crime does not bear the guilt of his parent. As the child conceived out of rape and incest is still an innocent life, procured abortion remains a grave crime.
The Church does not dismiss the difficulties that women face when they are in such tragic situations. Rather it emphasizes that "[E]ven in the most difficult circumstances human freedom is capable of extraordinary acts of sacrifice and solidarity to welcome the life of a new human being." 
Comfort for Women Who Have Had Abortions
In his encyclical Evangelium Vitae John Paul II addressed women who have had abortions:
I would now like to say a special word to women who have had an abortion. The Church is aware of the many factors which may have influenced your decision, and she does not doubt that in many cases it was a painful and even shattering decision. The wound in your heart may not yet have healed. Certainly what happened was and remains terribly wrong. But do not give in to discouragement and do not lose hope. Try rather to understand what happened and face it honestly. If you have not already done so, give yourselves over with humility and trust to repentance. The Father of mercies is ready to give you his forgiveness and his peace in the Sacrament of Reconciliation. To the same Father and his mercy you can with sure hope entrust your child. With the friendly and expert help and advice of other people, and as a result of your own painful experience, you can be among the most eloquent defenders of everyone's right to life. Through your commitment to life, whether by accepting the birth of other children or by welcoming and caring for those most in need of someone to be close to them, you will become promoters of a new way of looking at human life. 
1. CCC, 2270
2. CCC, 2273
3. Evangelium Vitae, 57
4. CCC, 2272
5. Evangelium Vitae, 58
6. CCC, 2356
7. Pope Benedict XVI, Speech to Participants at the International Congress "Oil on the Wounds: A Response to the Aftermath of Abortion and Divorce," (April 5, 2008)
6. Evangelium Vitae, 99