My heart goes out to you and your wife. A difficult pregnancy is not something any woman looks forward to, especially when she’s experiencing emotional distress. But there is never justification for the deliberate interruption of the conjugal act through the use of contraception. Humanae Vitae clearly states
[T]hat the direct interruption of the generative process already begun and, above all, all direct abortion, even for therapeutic reasons, are to be absolutely excluded as lawful means of regulating the number of children…Similarly excluded is any action which either before, at the moment of, or after sexual intercourse, is specifically intended to prevent procreation—whether as an end or as a means.
Neither is it valid to argue, as a justification for sexual intercourse which is deliberately contraceptive, that a lesser evil is to be preferred to a greater one…Though it is true that sometimes it is lawful to tolerate a lesser moral evil in order to avoid a greater evil or in order to promote a greater good, it is never lawful, even for the gravest reasons, to do evil that good may come of it—in other words, to intend directly something which of its very nature contradicts the moral order, and which must therefore be judged unworthy of man, even though the intention is to protect or promote the welfare of an individual, of a family or of society in general. Consequently, it is a serious error to think that a whole married life of otherwise normal relations can justify sexual intercourse which is deliberately contraceptive and so intrinsically wrong. (Humanae Vitae 14)
If you need to avoid pregnancy due to the difficulties your wife encounters during them, then the thing to do is investigate Natural Family Planning. There are many resources for this online, such as those provided by the Couple to Couple League, International (www.ccli.org).