[1][2] In accordance with the Church's teachings regarding sexual behavior, NFP excludes the use of other methods of birth control, which it refers to as "artificial contraception".
[3] When used to avoid pregnancy, couples may engage in sexual intercourse during a woman's naturally occurring infertile times such as during portions of her ovulatory cycle.
Natural family planning has shown very weak and contradictory results in pre-selecting the sex of a child, with the exception of a Nigerian study at odds with all other findings.
"[7] The Manichaeans (the group the early church father St. Augustine wrote of and considered to be heretics) believed that it was immoral to create any children, thus (by their belief system), trapping souls in mortal bodies.
"[9] Saint Thomas Aquinas wrote in his Summa Contra Gentiles: "Hence it is clear that every emission of the semen is contrary to the good of man, which takes place in a way whereby generation is impossible; and if this is done on purpose, it must be a sin.
[12] Documented attempts to prevent pregnancy by practicing periodic abstinence do not appear again until the mid-19th century, when various calendar-based methods were developed "by a few secular thinkers".
[16] In the 1920s Japanese gynecologist Kyusaku Ogino and Austrian Hermann Knaus independently made the discovery that ovulation occurs about fourteen days before the next menstrual period.
Smulders published his work with the Dutch Roman Catholic medical association, and this was the official rhythm method promoted over the next several decades.
Wilhelm Hillebrand, a Catholic priest in Germany, developed a system for avoiding pregnancy based on basal body temperature.
[14] Some historians consider two speeches delivered by Pope Pius XII in 1951[21] to be the first unequivocal acceptance of periodic abstinence by the Catholic Church.
Billings and several other physicians studied this sign for a number of years, and by the late 1960s had performed clinical trials and begun to set up teaching centers around the world.
Beyond that the council of bishops was told to leave to the Pontifical Commission on Birth Control the task of advising Pope Paul VI on the issue.
[23] Humanae Vitae, published in 1968 by Pope Paul VI, addressed a pastoral directive to scientists: "It is supremely desirable... that medical science should by the study of natural rhythms succeed in determining a sufficiently secure basis for the chaste limitation of offspring."
[20] During the following decade, other now-large Catholic organizations were formed: Family of the Americas(1977),[24] and the Creighton Model as part of the Pope Paul VI Institute (1985),[25] both mucus-based systems of NFP.
Some Catholic sources consider couples that violate the religious restrictions associated with natural family planning to not be NFP users.
[34] Of all American women surveyed nationally in 2002, only 0.9% were using "periodic abstinence" (defined as "calendar rhythm" and "natural family planning") compared to 60.6% using other contraceptive methods.
[40] Some fundamental Christians espouse Quiverfull theology, eschewing all forms of birth control, including natural family planning.
[44][45][46] Proponents justify this classification system by saying that NFP has unique characteristics not shared by any other method of birth regulation except for abstinence.
Clinical studies by the Guttmacher Institute found that periodic abstinence resulted in a 25.3 percent failure under typical conditions, though it did not differentiate between symptom-based and calendar-based methods.
[54] Computerized fertility monitors, such as Lady-Comp, may track basal body temperatures, hormonal levels in urine, changes in electrical resistance of a woman's saliva, or a mixture of these symptoms.
[56] A study by the World Health Organization involving 869 fertile women from Australia, India, Ireland, the Philippines, and El Salvador found that 93% could accurately interpret their body's signals regardless of education and culture.
They insisted that "a Catholic Christian is not free to form his conscience without consideration of the teaching of the magisterium, in the particular instance exercised by the Holy Father in an encyclical letter.
[66] In 2003, the BBC's Panorama claimed that Church officials have taught that HIV can pass through the membrane of the latex rubber from which condoms were made.
In an interview on Dutch television in 2004, Belgian Cardinal Godfried Danneels argued that the use of condoms should be supported to prevent AIDS if sex with a person infected with HIV should take place, though it is to be avoided.
[71] Mumford gives as an example the citation made by dissident theologian August Bernhard Hasler of a minority report co-authored by Pope John Paul II prior to his papacy: If it should be declared that contraception is not evil in itself, then we should have to concede frankly that the Holy Spirit had been on the side of the Protestant churches in 1930 (when the encyclical Casti connubii was promulgated), in 1951 (Pius XII's address to the midwives), and in 1958 (the address delivered before the Society of Hematologists in the year the pope died).
The fact can neither be denied nor ignored that these same acts would now be declared licit on the grounds of principles cited by the Protestants, which popes and bishops have either condemned or at least not approved.
[73] According to M. R. Gagnebet, though the encyclical Humanae vitae is considered by some to be a non-infallible document, "the doctrinal authority of the Pope and the Bishops is not limited to infallible teaching.
The Reformed theologian John Piper's Desiring God ministry states of NFP, "There is no reason to conclude that natural family planning is appropriate but that 'artificial' (non-abortive) means are not.