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A prospective multicentre trial of the ovulation method of natural family planning. Pt. 2. The effectiveness phase.
Fertility and Sterility. 1981 Nov; 36(5):591-98.A 5 country prospective study was undertaken to determine the effectiveness of the ovulation method of natural family planning. 869 subjects of proven fertility from 5 centers (Auckland, Bangalore, Dublin, Manila, and San Miguel) entered the teaching phase of 3-6 cycles; 765 (88%) completed the phase. 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. Cycle characteristics included: 1) average duration of the fertile period of 9.6 days, 2) mean of 13.5 days occurred from the mucus peak to the end of the cycle, 3) a mean of 15.4 days of abstinence was required, and 4) a mean of 13.1 days of intercourse was permitted. Almost all women were able to identify the fertile period by observing their cervical mucus but pregnancy rates ranged from 27.9 in Australia and 26.9 in Dublin to 12.8 in Manila. Continuation was relatively high ranging from 52% in Auckland to 74% in Bangalore.