What reasons justify avoiding pregnancy?

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).
Vademecum for Confessors 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).