The basic idea here is to appeal to the principle of double effect, but it is not at all clear that pre-eclampsia constitutes a sufficient condition for invoking that principle. For one thing, pre-eclampsia, in itself, simply isn't as life-threatening as the principle would require. Second, even if the condition were life-threatening, it isn't clear that the principle would apply to this sort of case.
In a statement released in late June, the United States Bishops' Committee on Doctrine drew a distinction between kinds of cases based upon the nature of the threat to the mother's life. They gave the following examples to illustrate the distinction. In one sort of case, we are to imagine a woman whose organs are experiencing problems as a consequence of the added burden of pregnancy; in the other sort of case, we are to imagine a woman with cancer of the uterus. According to the document, the former case is not sufficient to render licit an abortion to save the life of the mother, because to remove the fetus in order to save the mother's life is a direct killing of a human being in order to save the life of another, and this is not permitted by Catholic moral theory: we may not do wrong in order to bring about good. In the latter case, the removal of the cancerous uterus is what is directly intended, and the death of the fetus, though foreseeable, is not intended, so the principle of double effect applies. It is difficult to avoid drawing the inference that, when it is the mother's own organs that threaten her life, they may be removed even if doing so kills another human being who is lurking therein; but when it is another human being who threatens the mother's life, that human being may not be killed, even if it is hiding in one of the mother's organs.
The case of ectopic pregnancy, in light of the directive from the Committee on Doctrine, is particularly interesting. Some Catholics have thought that an ectopic pregnancy is a clear case in which abortion to save the life of the mother would be licit under the principle of double-effect. But clearly not, if the distinction drawn by the Bishops' Committee is correct. In the case of ectopic pregnancy there is nothing organically wrong with the fallopian tube, so it is not analogous to the case of the cancerous utuerus. Instead, the fetus has become lodged in the fallopian tube, and the only way to save the life of the mother is to either (a) remove the fetus from the fallopian tube, which would be a clear case of a direct abortion and hence illicit; or (b) remove the entire fallopian tube, with the fetus in it. It is this latter scenario, (b), that those Catholics who defend this procedure as licit point to as the case covered by the principle of double effect. Their thinking is that the intention is to save the life of the mother, not to kill the fetus, and they save the life of the mother by removing, not the fetus, but the fallopian tube. The death of the fetus is, of course, foreseeable in this scenario, but since it is not intended, these folk reason, it is licit because covered by double effect. But according to the distinction drawn by the Bishops the removal of the fallopian tube cannot be seen as anything other than the removal of the fetus, since the fallopian tube itself is in no need of being removed, and certainly would not need to be removed if there were not a fetus lodged in it. If a stone had become lodged in the fallopian tube there is little doubt that the procedure would be referred to as the removal of the stone and not as the removal of a fallopian tube that just happened to have a stone in it.
Fortunately ectopic pregnancy is extremely rare, occurring in less than 2% of all pregnancies, though it is, apparently, on the rise: it has increased sixfold since 1970. Most patients that present with ectopic pregnancy have no identifiable risk factor, so the temptation to blame the rise of the condition on increasing use of fertility treatments (also frowned upon by Catholic moral theory) must be resisted. However, I won't resist the temptation to report the following (from MedScape):
One study has demonstrated that infertility patients with luteal phase defects have a statistically higher ectopic pregnancy rate than patients whose infertility is caused by anovulation. The risk of ectopic pregnancy and heterotopic pregnancy (ie, pregnancies occurring simultaneously in different body sites) dramatically increases when a patient has used assisted reproductive techniques to conceive, such as in vitro fertilization (IVF) or gamete intrafallopian transfer (GIFT). In a study of 3000 clinical pregnancies achieved through in vitro fertilization, the ectopic pregnancy rate was 4.5%, which is more than double the background incidence. Furthermore, studies have demonstrated that up to 1% of pregnancies achieved through IVF or GIFT can result in a heterotopic gestation, compared to an incidence of 1 in 30,000 pregnancies for spontaneous conceptions.So apparently the only method of saving the life of the mother in the case of an ectopic pregnancy is not actually morally licit, according to the most recent statement by the United States Bishops. Not all Catholics commentators have argued that it is licit, but those who have will now have to rethink their arguments.