From Humanae Vitae:


Issuing of Humanae Vitae (On Human Life)


Pope Paul VI had maintained a panel of experts established by Pope John XXIII to consider the morality of contraceptive use. Due partly to media leaks and partly to the cultural milieu, many Catholics and other Americans expected the pope to permit at least limited use of birth control. Instead, Paul confirmed traditional Church teaching that prohibited all forms of artificial birth control (natural methods such as rhythm and sympto-thermal were permitted). The negative reaction of a large number of American Catholics, including priests and theologians, was an unprecedented reception for a papal encyclical in the United States.


Humana Vitae * Wikipedia


Pope Paul VI, Of Human Life-Humanae Vitae


J. Smith, Humanae Vitae: A Generation Later