Now that we've examined Pius XII's address to widwives, let's take a look at his address to the Directors of the Associations for Large Families of Rome and of Italy, given about seven years later.
"But you do not represent just any families at all; you are and represent large families, those most blessed by God and specially loved and prized by the Church as its most precious treasures. For these families offer particularly clear testimony to three things that serve to assure the world of the truth of the Church's doctrine and the soundness of its practice, and that redound, through good example, to the great benefit of all other families and of civil society itself."
"Wherever you find large families in great numbers, they point to: the physical and moral health of a Christian people; a living faith in God and trust in His Providence; the fruitful and joyful holiness of Catholic marriage."
Aha! We haven't yet found a decent definition of "serious" and "grave" reasons for avoiding conception of a/another child, however what we have is praise for large families. They are "most blessed by God and specially loved and prized by the Church as its most precious treasures" and they give "clear testimony to three things that serve to assure the world of the truth of the Church's doctrine and the soundness of its practice". What are these doctrines and practices? Unfettered and unimpeded fecundity. It is very clear from reading this that the Church wishes that all couples could have large families. This desire should be one of the teachings that inform people's consciences, which they must employ when discerning whether or not to periodically abstain from intercourse. There's are a couple catches to this, though. Firstly, we do not know exactly what Pius meant by "large". Secondly, we must be careful that we do not exalt fertility "beyond all measure", as the previous address warned. Erring on the side of caution, perhaps one could say that, all things being equal, families should grow to be as large as circumstances allow.
"Surely, one of the most harmful aberrations that has appeared in modern society with its pagan tendencies is the opinion of those who are eager to classify fruitfulness in marriage as a 'social malady,' and who maintain that any nation that finds itself thus afflicted must exert every effort and use every means to cure the disease. This is the basis for the propaganda that goes under the name of 'planned parenthood'; at times it is promoted by persons and organizations who command respect because of their positions in other fields, but who, unfortunately, have taken a stand in this matter which must be condemned." [emphasis mine]
"On the part of Catholics, We must urge the wide dissemination of the principle, firmly founded on truth, that the only way to protect the physical and moral health of the family and of society is through whole-hearted obedience to the laws of nature, or rather of the Creator, and most of all by fostering a sacred, heart-felt respect for them."
"In this matter, everything depends on the intention. You can multiply laws and make the penalties heavier; you can give irrefutable proofs of the stupidity of birth-control theories and of the harm that comes from putting them into practice; but as long as there is no sincere determination to let the Creator carry on His work as He chooses, then human selfishness will always find new sophistries and excuses to still the voice of conscience (to the extent it can), and to carry on abuses."
"In this matter, everything depends on the intention." The Church can, and has put, laws in place forbidding artificial birth control, but laws alone will not change behavior. Those who intend to act contraceptively will find ways to do so within the letter of the law while entirely ignoring its spirit. Even periodic abstinence can be used contraceptively. We can either work with God or against Him. If we wish to work against Him, we can start by guiding our lives by a contraceptive mentality. That's also a sure way to be a bad Catholic. Good Catholics should be open to life and encourage such openness in those around them. Note that begrudging acceptance of children is not true openness to life. Put more succintly, one should accept children joyfully and gratefully and only postpone conception with regret. On a side note, those "who are eager to classify fruitfulness in marriage as a 'social malady'" will not be pleased with these statistics.
"Now the value of the testimony offered by the parents of large families lies not only in their unequivocal and forceful rejection of any deliberate compromise between the law of God and human selfishness, but also in their readiness to accept joyfully and gratefully these priceless gifts of God-their children – in whatever number it may please Him to send them."
"This kind of attitude frees married couples from oppressive anxieties and remorse, and, in the opinion of outstanding doctors, creates the ideal psychological conditions for the healthy development of children born of the marriage. For, right at the beginning of these new lives, it eliminates all those worries and disturbances that can so easily leave physical or psychological scars on the mother or child."
I'm not certain what "oppressive anxieties and remorse" and "worries and disturbances" he's referring to, but I can safely say that he believes that free acceptance of multiple children is physically and psychologically better (than small families) for all involved. I'd like to see some proof of that, but for now I'll humor His Holiness and take his word for it. 😉 If anyone can to studies that support Pius' claims I'd be grateful.
"Again, good common sense has always and everywhere looked upon large families as a sign, a proof, and a source of physical health, and history makes no mistake when it points to violation and abuse of the laws governing marriage and procreation as the primary cause of the decay of peoples."
"Far from being a 'social malady,' large families are a guarantee of the moral and physical health of a people. Virtues flourish spontaneously in homes where a baby's cries always echo from the crib, and vice is put to flight, as if it has been chased away by the childhood that is renewed there like the fresh and invigorating breath of spring."
Far from being detrimental to societies, large families are a sign of their health. As families shrink, vices grow and societies crumble. Or at least, so says Pius. Some support for this claim would have been helpful.
"So let the weak and selfish take their example from you; let the nation continue to be loving and grateful toward you for all the sacrifices you have taken upon yourselves to raise and educate its citizens; just as the Church is pleased with you for enabling her to offer, along with you, ever healthier and larger groups of souls to the sanctifying activity of the divine Spirit."
The next time someone asks you if Catholics are supposed to have twelve kids, tell them, "If we are able, yes." Obviously, those pesky "serious" and "grave" reasons apply here and determine ability, though. To choose to have a small family, i.e., to have a small family not because of circumstances beyond one's ability to control, is to be "weak and selfish". In other words, "I just don't want a lot of kids" isn't a valid reason for a small family.