You cannot understand the distinction between direct abortion and indirect abortion unless you understand the three fonts of morality, especially the moral object. See my previous articles on this topic:
A summary of the three fonts of morality:
1. intention: the end intended by the subject (the person who acts).
This font is the reason or purpose for choosing the act. The intended end is in the subject, not in the act.
2. moral object: the end, in terms of morality, toward which the knowingly chosen act is inherently ordered.
This font includes the objective act that is intentionally (deliberately, voluntarily, knowingly) chosen by the person, and its inherent moral meaning before God, which is determined by its moral object. The intrinsic ordering of the chosen objective act toward its moral object is what makes the act intrinsically evil or intrinsically good (not the attainment of the moral object). The knowing and deliberate choice of any objective act by the person includes a choice of the act, and its inherent moral meaning, and its moral object. The moral object is in the objective act, not in the subject.
The deliberate and knowing choice of an act with an evil moral object is the choice of an immoral type of act, an act that is evil by its very nature. The moral nature of an act is also called the moral species, which is the type of act in terms of morality. Nothing can justify the choice of an intrinsically evil act.
Most Catholics, most priests, most religious, even most theologians, do not understand the moral object.
3. circumstances: the morality of this font is based on the moral weight of the reasonably anticipated good and bad consequences of the chosen act for all persons affected by the act. If the reasonably anticipated bad consequences morally outweigh the reasonably anticipated good consequences — even if those bad consequences are not intended — the choice of such an act is always a sin. For it is always a sin to choose to act knowing that your act will do more harm than good.
To be moral, all three fonts of morality must be good. Any one bad font makes the act always a sin, for as long as any one font remains bad. Any one gravely disordered font makes the act always an objective mortal sin. Two good fonts cannot cause one bad font to become good, nor can two good fonts make the act moral.
Abortion is called direct because every intrinsically evil act has a direct relationship between its moral object and the inherent moral meaning of the act. Every intrinsically evil act is inherently ordered, by the very nature of the chosen act, towards an evil proximate (morally-immediate) end, which is its moral object.
Now it does not matter if the intentionally-chosen act has a good intended end, such as saving the life of the mother. That good end is in the first font, and cannot cause the second font to become moral. A good intended end cannot justify the choice of an evil means.
Direct abortion is intrinsically evil and always gravely immoral, whether chosen as an end in itself, because the woman does not wish to be pregnant, or as a means to another end, because the termination of the pregnancy will reduce the strain on an ill woman’s health. The moral object remains the same, whether the abortion is an end or a means, and therefore the act remains intrinsically evil.
Abortion is called indirect when the chosen act is inherently ordered, not toward the death of the prenatal, but toward a good, such as the health of the mother. Then the death of the prenatal is only a bad consequence in the third font of the morality. If the intention is only good, and the moral object is only good, and the good consequences are not outweighed by the weighty bad consequence of the death of an innocent prenatal, then indirect abortion can be moral. But indirect abortion is not always moral; it is only moral if all three fonts of morality are good.
Now it is important to understand that the death of the prenatal is ALWAYS a bad consequence; both direct abortion and indirect abortion have that bad consequence. So the fact that the death of the prenatal is a bad consequence in the third font, even an unintended bad consequence, does NOT prove that the abortion is indirect. Only a moral analysis of the intentionally chosen act, its inherent moral meaning, and its moral object can determine if the act is direct abortion or indirect abortion.
For example, if a woman has cancer of the uterus, and is in the early stages of pregnancy, it might not be possible to save the life of the prenatal. If the cancer is treated by removing the cancerous uterus, the prenatal dies. If the cancer is not treated, and it advances and kills the mother prior to the viability of the child, then both mother and prenatal die. In such a case, the act of removing the cancerous uterus is an example of a moral indirect abortion. The chosen act is directly ordered toward saving the life of the mother, not by means of the death of the prenatal, but as the morally-immediate end of the chosen act. Therefore, the death of the prenatal occurs as a result of the treatment directed at saving a life, and so the death of the prenatal is not the moral object.
If the health or life of the mother is protected or saved by means of the death of the prenatal, then that death is the moral object, and the good effect on the health or life of the mother is in the third font of consequences, not in the moral object. When the death of the prenatal is in the moral object, then the act is direct abortion. When the death of the prenatal is only in the consequences, and not at all in the moral object, then the abortion is indirect.
Suppose that the death of a prenatal due to a deliberate choice is an unintended bad consequence. Does this imply that the act is indirect abortion, not direct abortion? No, it does not. Abortion willed as an end is an intended bad consequence. But the Magisterium teaches that abortion is gravely immoral, whether willed as an end or as a means. Therefore, it is not sufficient, for the act to be moral, for the death of the prenatal to be an unintended bad consequence. The chosen act must not be inherently ordered toward that death in its moral object, not even as a means to a good end. The end does not justify the means.
We can distinguish between the moral object and other ends, such as the end of the first font (intended end) and the ends of the third font (the end results or consequences) quite simply. The moral object of an act is the morally-immediate end toward which the chosen act is intrinsically directed. If someone claims that a result of the act, “C”, is the moral object, but result “C” is obtained by means of another end, “B”, then “C” is not the moral object; instead “B” is the moral object. The moral object is always the morally-immediate end, i.e. the proximate end, toward which the chosen act is ordered by its very nature.
If the life of the mother is saved directly and immediately by the chosen act, and not by means of the death of the prenatal, then the act has the saving of her life as its moral object (making the act indirect abortion). If the life of the mother is saved by means of the death of the prenatal, then that death is the proximate (morally-immediate) end and therefore the moral object (making the act direct abortion).
Direct abortion is intrinsically evil and always gravely immoral. Direct abortion is never justified, not even to save the life of the mother. Neither a good intention, nor dire circumstances, can justify the deliberate and knowing choice of any intrinsically evil act.