The effectiveness of fertility awareness, as of most forms of
contraception,
can be assessed two ways. Perfect use or
method effectiveness rates only include people who follow all observational rules, correctly identify the fertile phase, and refrain from unprotected intercourse on days identified as fertile.
Actual use, or
typical use effectiveness rates are of all women relying on fertility awareness to avoid pregnancy, including those who fail to meet the “perfect use” criteria. Rates are generally presented for the first year of use.
[31] Most commonly the
Pearl Index is used to calculate effectiveness rates, but some studies use
decrement tables.
[32] The failure rate of fertility awareness varies widely depending on the system used to identify fertile days, the instructional method, and the population being studied. Some studies have found actual failure rates of 25% per year or higher.
[33][34][35] At least one study has found a failure rate of less than 1% per year with continuous intensive coaching and monthly review,[36] and several studies have found actual failure rates of 2-3% per year.
[37][38][29][39]
When used correctly and consistently with ongoing coaching, some studies have shown some forms of FA to be 99% effective,[40][41][36][42] the same as oral contraceptives.
[43]