Blessed Pope Paul VI in his 1968 encyclical Humanae Vitae (HV) presented the perennial Catholic teaching that "artificial birth control" was always "intrinsically wrong." He did, however, defend the use of what's called natural family planning (NFP) when serious reasons applied.
The natural method of spacing births called NFP, when done for unselfish motives, is morally accomplished by simply avoiding marital relations during the woman's fertile period. An obsolete form of NFP, called the rhythm or calendar method, attempts to predict a woman's fertility based only on the number of days since the start of her last feminine cycle. In a typical 28-day cycle, ovulation occurs around day 14. To avoid pregnancy, the couple would simply abstain from marital relations for about five days before ovulation until a couple days after ovulation. The lead time is owing to the lifespan of semen once inside the woman. This method isn't very accurate as many women don't have consistent monthly cycles.
Another form of NFP is the highly accurate sympto-thermal method that indicates fertility based on a woman's temperature, cervix position and mucus secretions. During fertility the woman's basal temperature rises about a half a degree, her cervix rises almost an inch and the flow of mucus is noticeably heavier. To avoid pregnancy the couple, once again, would simply avoid marital relations from five days prior to the crossing of these symptoms until a couple days afterward. Two other major forms of NFP include the Creighton and Marquette Models.
These methods are considered 99 percent effective at spacing births and are recommended by Catholic healthcare professionals. The argument, therefore, that artificial forms of contraception must be employed in grave circumstances isn't scientific. Most contraceptives are far less effective than the sympto-thermal form of NFP.
The use of NFP doesn't constitute a frustrated sexual act that immorally separates the unitive and procreative aspects of sex. Natural Family Planning is, therefore, not objectively or intrinsically immoral. Because the ends of marriage include procreation, however, Paul VI taught in HV that the use of NFP is immoral unless there are "well-grounded reasons for spacing births."
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