The Catechism of the Catholic Church explains, "A particular.aspect of [the fecundity of marriage] concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children" (CCC 2368). But the Catechism does not explicitly define what constitutes "just reasons." Instead, proper determination is left up to the couple: "It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood" (CCC 2368).

However, the language used in Church documents may be somewhat helpful. For example, Gaudium et Spes states,

[C]ertain modern conditions often keep couples from arranging their married lives harmoniously, and . . . they find themselves in circumstances where at least temporarily the size of their families should not be increased. As a result, the faithful exercise of love and the full intimacy of their lives is hard to maintain. (GS 51)

Humanae Vitae says that a couple may morally space births if there are well-grounded reasons "arising from the physical or psychological condition of husband or wife, or from external circumstances" (HV 16).

Ultimately, each couple must determine for themselves whether "just reasons" truly exist. Vademecum for Confessors Concerning Some Aspects of the Morality of Conjugal Life explains, "Certainly it is a duty of married couples—who, for that matter, should seek appropriate counsel—to deliberate deeply and in a spirit of faith about the size of their family, and to decide the concrete mode of realizing it, with respect for the moral criteria of conjugal life" (2.3).

Parents should regard as their proper mission the task of transmitting human life and educating those to whom it has been transmitted. They should realize that they are thereby cooperators with the love of God the Creator, and are, so to speak, the interpreters of that love. Thus they will fulfill their task with human and Christian responsibility, and, with docile reverence toward God, will make decisions by common counsel and effort. Let them thoughtfully take into account both their own welfare and that of their children, those already born and those which the future may bring. For this accounting they need to reckon with both the material and the spiritual conditions of the times as well as of their state in life. Finally, they should consult the interests of the family group, of temporal society, and of the Church herself. The parents themselves and no one else should ultimately make this judgment in the sight of God. But in their manner of acting, spouses should be aware that they cannot proceed arbitrarily, but must always be governed according to a conscience dutifully conformed to the divine law itself, and should be submissive toward the Church’s teaching office, which authentically interprets that law in the light of the Gospel. That divine law reveals and protects the integral meaning of conjugal love, and impels it toward a truly human fulfillment. (GS 50)

You can follow your doctor’s advice. Legitimate medical procedures and medications necessary for healing the body are not at odds with Church teaching. In his encyclical Humanae Vitae, Pope Paul VI stated: "[T]he Church does not consider at all illicit the use of those therapeutic means necessary to cure bodily diseases, even if a foreseeable impediment to procreation should result there from—provided such impediment is not directly intended for any motive whatsoever" (HV 15).

If NFP is not reliable for any reason, you may find the only moral method for regulating births to be abstinence. However, there are methods of breastfeeding through which the mother will generally experience an extended period of infertility. For more on this, I recommend that you consult an NFP expert.

God said, "Be fruitful and multiply, and fill the earth . . ." (Gen. 1:28). The Catechism of the Catholic Church teaches, "Sacred Scripture and the Church’s traditional practice see in large families a sign of God’s blessing and the parents’ generosity" (CCC 2373). So does this mean that having a large family is every married couple’s moral responsibility? The answer is no. In fact, family size is not an issue of morality at all—but both openness to children and responsible family planning are.

It is important to understand that engendering and raising children is one of the primary purposes of marriage. The Catechism explains, "The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring" (CCC 1601). "Fecundity is a gift, an end of marriage, for conjugal love naturally tends to be fruitful. A child does not come from outside as something added on to the mutual love of the spouses, but springs from the very heart of that mutual giving, as its fruit and fulfillment" (CCC 2366).

That said, couples have a moral obligation to plan their families responsibly.

A particular.aspect of this responsibility concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood. (CCC 2368)

Ultimately, family size should be determined by responsible procreation in cooperation with the love of God the Creator (cf. CCC 2367).

Homologous artificial insemination and fertilization are generally immoral. The Catechism of the Catholic Church specifically defines and addresses these techniques:

Techniques involving only the married couple (homologous artificial insemination and fertilization) are perhaps less reprehensible [than techniques that require the intrusion of a donor or surrogate] yet remain morally unacceptable. They dissociate the sexual act from the procreative act. The act that brings the child into existence is no longer an act by which two persons give themselves to one another but one that "entrusts the life and identity of the embryo into the power of doctors and biologists and establishes the domination of technology over the origin and destiny of the human person. Such a relationship of domination is in itself contrary to the dignity and equality that must be common to parents and children." (CCC 2377; cf. Donum Vitae 5)

That said, the Church does recognize an exception "for those cases in which the technical means is not a substitute for the conjugal act but serves to facilitate and to help so that the act attains its natural purpose" (DV 6).

You may be confusing infertility with impotence. Infertility (the inability to procreate children) is not an impediment to marriage; permanent and irreversible impotence (the inability to consummate a marriage through marital relations) is an impediment. Impotence that is known at the time of the marriage to be permanent and irreversible is a barrier to marriage, because the couple must be capable of consummating their marriage. If the couple has reason to assume that the impotence can be treated or reversed, they may get married.