NFP enables spouses to space births according to the naturally-occurring phases of fertility and infertility in the menstrual cycle. The Church has accepted this innovation of the 20th century as a morally acceptable means of spacing and limiting births in married life. The contemporary Church document, Humanae vitae, which articulated the reasons why NFP is acceptable, uses the words “serious” and “grave” to indicate the distinctions which spouses need to consider as they seek to plan their families according to God’s will. Catholic couples need to understand the meaning behind these words.

Historical Overview
The Church has always recognized the legitimacy of abstaining from sexual intercourse when both spouses consent for a limited time and for religious reasons (cf. 1 Cor. 7:5). When Pius XI condemned contraception in his encyclical on marriage, Casti connubii (Dec 31, 1930), he did not address the licitness of the Rhythm method which had only recently been discovered but did allow married couples the use of their conjugal rights “in the proper manner” when new life could not be brought forth either because of timing or defects of nature (no. 59). It was not until Pius XII that explicit pronouncements were made. By that time the Basal Body Temperature method (BBT) was becoming increasingly known and used among Catholics.

Pius XII, in an address to Italian midwives in October 1951, declared licit the use of the sterile period for serious reasons, but if the couple was confining intercourse to those days exclusively, their conduct needed to be examined. In that case, it was not enough to be ready to accept a pregnancy if one should occur. For the practice to be moral there must be serious reasons independent of the couple’s goodwill. Otherwise to do so “would be a sin against the very meaning of conjugal life.” At the same time, Pius XII advised midwives to obtain a thorough knowledge of the biological and technical aspects of the theory.

Among the serious reasons for use even for an indefinite period, Pius XII cited “medical, eugenic, economic and social implication.” [1] Only one month later in another address, the pope affirmed “the legitimacy and, at the same time, the limits–in truth very wide–of a regulation of offspring, which, unlike so-called ‘birth control,’ is compatible with the law of God,” and he hoped that science would provide a more secure basis for the method. [2]

The advent of the anovulant pill in the 1960s and pressure from within the Church itself to change its teaching on contraception in the name of enhancing the unitive dimension of marriage led to lively debates in Vatican Council II. While Pope Paul VI reserved the question of whether the anovulant pill was a contraceptive until after the Council, the pastoral constitution on the Church in the modern world, Gaudium et Spes, reaffirmed that “marriage and conjugal love are by their nature ordered to the procreation and education of children” (no. 48), and that the aim and meaning of conjugal life is to cooperate with the Creator in enlarging God’s family. As cooperators with the Creator, they are “interpreters of his love” (no. 50).

Spouses will thoughtfully take into account both their own welfare and that of their children, those already born and those which may be foreseen. For this accounting, they will reckon with both the material and spiritual conditions of the times as well as of their state of life. Finally, they will consult the interests of the family group, of temporal society, and of the Church herself (See GS, no.50).

Grave and Serious
Paul VI in his encyclical Humanae vitae (1968), while condemning the use of all contraceptive methods for even grave (gravia) reasons, declared licit the recourse to the infertile periods if the spouses have good (just and seria) reasons to postpone even indefinitely another pregnancy (HV, no.16 &10; the language here is similar to Gaudium et Spes, no.10). But first, those spouses are commended who, with prudent deliberation and generosity, choose to accept a large family. The spouses are to consider their responsibilities towards God, themselves, the family, and human society. Each of these factors may be taken into account in the right order in determining “serious and just reasons.”

In other words, the spouses are to discern together first, what is God’s plan for their family here and now, then their own physical and emotional resources for accepting another child, the needs of other family members, and lastly the good of the human society in which they live. The pope gives special encouragement to scientists to perfect the natural methods (HV, no.24), declaring that the discipline of chastity exercised in periodic continence enhances married life provided the spouses value the true blessings of the family (HV, no.21).

John Paul II
John Paul II is faithful to the guidelines of Humanae vitae. In the Apostolic Exhortation, Familiaris consortio, he calls the fundamental task of the family “to serve life, to actualize in history the original blessing of the Creator–of transmitting by procreation the divine image from person to person” (FC, no.28). The Holy Father praises large families [3]; however, he also states,

. . . the fruitfulness of conjugal love is not restricted solely to the procreation of children…it is enlarged and enriched by all those fruits of moral, spiritual and supernatural life which the father and mother are called to hand on to their children, and through the children to the Church and to the world. (FC, no.28)