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).