fertile, but this remained the minority view for the remainder of the 19th century. Knaus–Ogino or rhythm method In 1905 Theodoor Hendrik van de Velde, a Dutch gynecologist, showed that women only ovulate once per menstrual cycle. In the 1920s, Kyusaku Ogino, a Japanese gynecologist, and Hermann Knaus, from Austria, working independently, each made the discovery that ovulation occurs about fourteen days before the next menstrual period. Ogino used his discovery to develop a formula for use in aiding infertile women to time intercourse to achieve pregnancy. In 1930, Johannes Smulders, a Roman Catholic physician from the Netherlands, used Knaus and Ogino's discoveries to create a method for avoiding pregnancy. Smulders published his work with the Dutch Roman Catholic medical association, and this was the official rhythm method promoted over the next several decades. In 1932 a Catholic physician, Dr. Leo J Latz, published a book titled The Rhythm of Sterility and Fertility in Women describing the method, and the 1930s also saw the first U.S. Rhythm Clinic (founded by John Rock) to teach the method to Catholic couples. Later 20th century to present In the first half of the 20th century, most users of the rhythm method were Catholic; they were following their church's teaching that all other methods of birth control were sinful. In 1968 the encyclical Humanae vitae included the statement, "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. Currently, many fertility awareness teachers consider the rhythm method to have been obsolete for at least 20 years. New attention was drawn to calendar-based methods in 2002, when the Institute for Reproductive Health at Georgetown University introduced the Standard Days Method. Designed to be simpler to teach and use than the older rhythm method, the Standard Days Method is being successfully integrated into family planning programs worldwide. Types and effectiveness Most menstrual cycles have several days at the beginning that are infertile (pre-ovulatory infertility), a period of fertility, and then several days just before the next menstruation that are infertile (post-ovulatory infertility). The first day of red bleeding is considered day one of the menstrual cycle. To use these methods, a woman is required to know the length of her menstrual cycles.