Fertility:
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. (Gaudium et Spes, #50)
Let them [husband and wife] 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. (Gaudium et Spes, #50)
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. (Gaudium et Spes, #50)