Yes, unless the operation is necessary and doesn't directly kill the fetus. (Some Catholics might consider that these cases do not constitute abortions. I haven't found clear official declarations one way or the other on this semantics issue.)
The Church teaches that the unborn are human persons like you and I. From the Catechism,
2270 Human life must be respected and protected absolutely from the moment of conception. From the first moment of his existence, a human being must be recognized as having the rights of a person - among which is the inviolable right of every innocent being to life.
Because the unborn are human persons, deliberate killing is almost always wrong.
2271 Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable. Direct abortion, that is to say, abortion willed either as an end or a means, is gravely contrary to the moral law.
However, there are some cases in which killing is not immoral. The principle of double effect explains why killing in self-defense is often not immoral.
2263 The legitimate defense of persons and societies is not an exception to the prohibition against the murder of the innocent that constitutes intentional killing. "The act of self-defense can have a double effect: the preservation of one's own life; and the killing of the aggressor. . . . The one is intended, the other is not."
 Unintentional killing is not morally imputable. But one is not exonerated from grave offense if, without proportionate reasons, he has acted in a way that brings about someone's death, even without the intention to do so.
Finally, there are some cases in which letting someone die can be a moral choice. Again, the principle of double effect explains this. For example, if the lives of two people are in danger, and only one can be saved. This applies in the case of abortion. From the Catholic Encyclopedia:
However, if medical treatment or surgical operation, necessary to save a mother's life, is applied to her organism (though the child's death would, or at least might, follow as a regretted but unavoidable consequence), it should not be maintained that the fetal life is thereby directly attacked. [...] The death of the child is not intended, and every reasonable precaution is taken to save its life; the immediate effect intended, the mother's life, is good — no harm is done to the child in order to save the mother — the saving of the mother's life is in itself as good as the saving of the child's life.
So, in summary, the Church teaches that:
- The unborn have "the rights of a person", including the right to live.
- It is morally permissible to let one person die to save another, if necessary. This includes life-saving operations on the mother that cause the fetus to die, as long as the fetus is not directly killed.
- It is morally permissible to kill an attacker to save another's life, if necessary.
- But it is not morally permissible to kill one non-attacker to save another.