Bishop Thomas Olmsted of Phoenix is now the third U.S. bishop to require a full course of natural family planning for couples who want to marry in a Catholic church. The other dioceses are Denver and Fargo. Most other dioceses only require an introduction to the method.

In a series of articles, Bishop Olmsted said artificial birth control is a grave wrong that is at the root of many of the problems afflicting American society, reported the Arizona Republic. He said: "marriage itself is gravely harmed" when a couple uses birth control, and that marital infidelity increases. He added: "The use of contraceptives is always morally evil, and many of them have harmful side effects as well."

Peggy Frei, who heads the Natural Family Planning Center of the Diocese of Phoenix, told the Arizona Republic that there are no statistics on how many people use natural family planning, but her office taught the full course to 422 people in 2004 and 2,469 individuals took the introductory class.

Once the full-course requirement is in place, it will teach about 2,000 couples a year, the average number of marriages conducted in the diocese since it was founded in 1969.

The American Academy of Family Physicians says that when couples are diligent, natural family planning is 90 to 98 percent effective.