Clinical studies by the Guttmacher Institute found that periodic abstinence resulted in a 25.3 percent failure under typical conditions, though it did not differentiate between symptom-based and calendar-based methods.
Natural family planning, the family planning methods approved by the Roman Catholic Church
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This precludes them from utilizing some forms of "natural birth control" such as the "Calendar-based contraceptive methods" which are relatively unobjectionable to other religious groups.