the Winnipeg Statement, in which the bishops recognized that a number of Canadian Catholics found it "either extremely difficult or even impossible to make their own all elements of this doctrine" (that of Humanae vitae). Additionally, in 1969, they reasserted the Catholic principle of primacy of conscience, a principle that they said should be properly interpreted. They insisted that "a Catholic Christian is not free to form his conscience without consideration of the teaching of the magisterium, in the particular instance exercised by the Holy Father (i.e., Pope) in an encyclical letter". According to the American Enterprise Institute, 78% of American Catholics say they believe the Church should allow Catholics to use birth control, though other polls reflect different numbers. According to Stephen D. Mumford, the Vatican's opposition towards birth control continues to this day and has been a major influence on United States policies concerning the problem of population growth and unrestricted access to birth control. However, in December 2018, in a responsum (a reply by a Curial department that is intended to settle a question or dispute, but that is not a papal document), the Congregation for the Doctrine of the Faith (CDF), under its Prefect, Cardinal Luis Ladaria Ferrer, S.J., stated that if the uterus can be found, with moral certainty, to not be able to ever carry a fertilized ovum to the point of viability, that a hysterectomy could be performed, because under that very narrow circumstance it is considered the removal of a failed organ and not per se a sterilization, since viability is not possible. If a hysterectomy is only done under this circumstance, it does not represent a shift in church teaching. Despite the Church's teaching on contraception, spacing out births by natural means is a behavior that remains in full communion with the Church. Many married couples employ natural family planning (NFP) in order to bolster the unitive aspect of the marital union without compromising the oppenness to life. Studies show a correlation between the practice of NFP and healthy spousal relationships in married couples who utilize this practice. Mainline Protestantism Author and FamilyLife Today radio host Dennis Rainey suggests four categories as useful in understanding current Protestant attitudes concerning birth control. These are the "children in abundance" group, such as Quiverfull adherents who view all birth control and natural family planning as wrong; the "children in managed abundance" group, which accept