Rice, F. J., Lanctot, C. A., & Garcia-Devesa, C. (1981). Effectiveness of the sympto-thermal method of natural family planning: an international study. International Journal of Fertility, 26(3), 222–230.
Key Results
The study indicates that 128 unplanned pregnancies were recorded, a rate of 7.47 conceptions per 100 women-years of exposure, and 16 of these pregnancies occurred when the couples were following the method strictly.” and “Only 16 pregnancies occurred when the couples were following instructions, giving a theoretical effectiveness of .93 pregnancies per 100 woman years using the Pearl formula. Couples trying to prevent any pregnancies had a failure rate of 4.13%, those only delaying a pregnancy had a failure rate of 14.83%. The sympto-thermal method used alone had a failure rate of 7.16% while use of contraceptive devices with sympto-thermal method had a failure rate of 8.19%.
Conclusion
Any valid comparative figure for NFP should be expressed in pregnancies per 1,300 cycles according to the modified Pearl index rather than 1,200 months. This gives a pregnancy rate of 7.66% as compared to 7.47% for 1,200 months.