A prospective multicentre trial of the ovulation method of natural family planning. II. The effectiveness phase.

A five-country prospective study was undertaken to determine the effectiveness of the ovulation method of natural family planning. After successful completion of a teaching phase of three cycles, 725 subjects entered a 13-cycle effectiveness phase and contributed 7514 cycles of observation. The overall cumulative net probability of discontinuation for the effectiveness study after 13 cycles was 35.6%, 19.6% due to pregnancy. Pregnancy rates per 100 woman-years calculated using the modified Pearl index were as follows: conscious departure from the rules of the method, 15.4; inaccurate application of instructions, 3.5; method failure, 2.8; inadequate teaching, 0.4; and uncertain, 0.5.