Natural family planning (NFP) comprises the family planning methods approved by the Catholic Church and some Protestant denominations for both achieving and postponing or avoiding pregnancy. 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". Periodic abstinence, the crux of NFP, is deemed moral by the Church for avoiding or postponing pregnancy for just reasons. 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. Various methods may be used to identify whether a woman is likely to be fertile; this information may be used in attempts to either avoid or achieve pregnancy. Effectiveness can vary widely, depending on the method used, whether the users were trained properly, and how carefully the couple followed the protocol. Pregnancy can result in up to 25% of the user population per year for users of the symptoms-based or calendar-based methods, depending on the method used and how carefully it was practised. 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. Because of these remarkable results, an independent study needs to be repeated in other populations. History Pre-20th century In ancient history, some Christian writers were against abstinence to prevent childbirth and some allowed it. Possibly the earliest Christian writing about periodic abstinence was by Clement of Alexandria. He wrote, "Let the Educator (Christ) put us to shame with the word of Ezekiel: 'Put away your fornications' [Eze. 43:9]. Why, even unreasoning beasts know enough not to mate at certain times. To indulge in intercourse without intending children is to outrage nature, whom we should take as our instructor." In the year 388, St. Augustine wrote against the Manichaeans: "Is it not you who used to counsel us to observe as much as possible the time when a woman, after her purification, is most likely to conceive, and to abstain from cohabitation at that time...?" 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. Augustine condemned them for their use of periodic abstinence