centers around the world. The Vatican II Constitution on the Church in the Modern World declared: "While not making the other purposes of matrimony of less account, the true practice of conjugal love, and the whole meaning of the family life which results from it, have this aim: that the couple be ready with stout hearts to cooperate with the love of the Creator and the Savior. Who through them will enlarge and enrich His own family day by day" (50). 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. 64 of the 68 Commission members who voted recommended allowing other means of contraception, but Paul VI determined otherwise. 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." This is interpreted as favoring the then-new, more reliable symptoms-based fertility awareness methods over the rhythm method. Just a few years later, in 1971, the first organization to teach a symptothermal method, using both mucus and temperature observations, was started. Now called Couple to Couple League International, this organization was founded by John and Sheila Kippley, lay Catholics, along with Dr. Konald Prem. During the following decade, other now-large Catholic organizations were formed: Family of the Americas(1977), and the Creighton Model as part of the Pope Paul VI Institute (1985), both mucus-based systems of NFP. Use of the term natural family planning to describe calendar-based methods is considered incorrect by the United States Conference of Catholic Bishops, which considers such methods "inaccurate". Some organizations have considered calendar-based methods to be forms of NFP. For example, in 1999 the Institute for Reproductive Health at Georgetown University developed the Standard Days Method (SDM), which is more effective than the rhythm method. SDM is promoted by Georgetown University as a form of natural family planning. Prevalence It is estimated that 2%–3% of the world's reproductive age population relies on periodic abstinence to avoid pregnancy. However, what portion of this population should be considered NFP users is unclear. Some Catholic sources consider couples that violate the religious restrictions associated with natural family planning to not be NFP users. There is little data on the worldwide