The Church holds that the motivations behind NFP should not be selfish or self-centered. In 1880, the Sacred Penitentiary reaffirmed the 1853 ruling and went slightly further. It suggested that, in cases where the couple was already practising artificial birth control and could not be dissuaded to cease attempting birth regulation, the confessor might morally teach them of periodic abstinence. Early 20th century In 1905, Theodoor Hendrik van de Velde, a Dutch gynecologist, showed that women only ovulate once per menstrual cycle. 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. Ogino used his discovery to develop a formula for use in aiding infertile women to time intercourse to achieve pregnancy. In 1930, John Smulders, a Roman Catholic physician from the Netherlands, used Knaus and Ogino's discoveries to create the rhythm method. Smulders published his work with the Dutch Roman Catholic medical association, and this was the official rhythm method promoted over the next several decades. While maintaining procreation as the primary function of intercourse, the December 1930 encyclical Casti connubii by Pope Pius XI gave recognition to a secondary—unitive—purpose of sexual intercourse. This encyclical stated that there was no moral stain associated with having marital intercourse at times when "new life cannot be brought forth". This referred primarily to conditions such as current pregnancy and menopause. In 1932, a Catholic physician 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. It was during this decade that Rev. Wilhelm Hillebrand, a Catholic priest in Germany, developed a system for avoiding pregnancy based on basal body temperature. Since 20th century A minority of Catholic theologians continued to doubt the morality of periodic abstinence. Some historians consider two speeches delivered by Pope Pius XII in 1951 to be the first unequivocal acceptance of periodic abstinence by the Catholic Church. The 1950s also saw another major advance in fertility awareness knowledge: Dr. John Billings discovered the relationship between cervical mucus and fertility while working for the Melbourne Catholic Family Welfare Bureau. 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