Che, Y., Cleland, J. G. and Mohamed, M. A. Periodic abstinence in developing countries: an assessment of failure rates and consequences. Contraception 69 (2004): 15-21.
Key Results
Correct knowledge of ovulation ranged from 8% to 91% (median 62%). 24% failure rate of periodic abstinence (pregnancy).
Conclusion
Couples who experienced PA failure were more likely than couples who experienced failure with another method to carry the pregnancy to term. Nevertheless, PA still contributes to one-sixth of all abortions (or miscarriages) following contraceptive failure.