Arévalo, M., Sinai, I., & Jennings, V. (1999). A fixed formula to define the fertile window of the menstrual cycle as the basis of a simple method of natural family planning. Contraception, 60(6), 357-360. https://doi.org/10.1016/s0010-7824(99)00106-7
Arévalo M, Sinai I, Jennings V. A fixed formula to define the fertile window of the menstrual cycle as the basis of a simple method of natural family planning. Contraception. 1999;60(6):357-360. doi:10.1016/s0010-7824(99)00106-7
Arévalo, Marcos, et al. "A fixed formula to define the fertile window of the menstrual cycle as the basis of a simple method of natural family planning." Contraception, vol. 60, no. 6, 1999, pp. 357-360.
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Days 8 to 19 as the fertile window can be an effective method
A fixed fertile window of cycle days 8 to 19 can serve as a highly effective family planning method for most women. Among women with 26 to 32 day cycles, the highest estimated daily pregnancy probability was about 0.007. The authors analyzed about 7,600 cycles from a World Health Organization study.
Key Findings
The Standard Days method counsels abstinence from unprotected intercourse on days 8 to 19 inclusive. Reanalysis suggested day 8 would add a margin of safety to the original day 9 start.
The window included peak day and the days just before it in 96%–98% of cycles. Coverage was about 80% three days after peak day and 34% eight days before it.
Although 25.7% of women had every cycle within 26 to 32 days, about 78% of cycles fell in that range. For these women the highest estimated daily pregnancy probability was about 0.007.
For women whose cycles all ranged from 24 to 34 days (59.3% of women, 4,079 cycles), the highest estimated daily probability was about 0.009. Of all cycles, 94.8% fell in this range.
The authors conclude the Standard Days method can be highly effective for most couples who choose it, and it still seems effective when all cycle lengths, 12 to 64 days, are included.
Interpretation
The paper is a theoretical analysis. It applied published day-specific pregnancy probabilities to about 7,600 cycles from the World Health Organization's Ovulation Method research in five diverse settings. Peak day, the last day of fertile-type mucus or wet sensation, served as a generally accepted stand-in for ovulation. The analysis included cycles with a clearly defined peak day. The authors identify women with 26 to 32 day cycles as the group for whom the method seems most effective. They expect a study in several countries to confirm the potential effectiveness findings.
RRM Context
Fertility awareness-based methods help couples identify the days on which conception is possible. The Standard Days method defines those days by a fixed calendar formula, while the Ovulation Method data behind this analysis locate them through peak day. The analysis also draws on an earlier study by Hilgers and colleagues. That study found that 97% of ovulations fall within 2 days on either side of peak day, and 38% on peak day itself.
Our editorial summary of this paper, not the article's abstract.
Abstract
A significant number of women worldwide use periodic abstinence as their method of family planning. Many of them use some type of calendar-based approach to determine when they should abstain from unprotected intercourse to avoid pregnancy; yet they often lack correct knowledge of when during their menstrual cycle they are most likely to become pregnant. A simple method of natural family planning (NFP) based on a fixed formula to define the fertile window could be useful to these women. This article reports the results of an analysis of the application of a fixed formula to define the fertile window. A large existing data set from a World Health Organization study of the Ovulation Method was used to estimate the theoretical probability of pregnancy using this formula. Information about the variable probability of pregnancy on different cycle days relative to ovulation also was considered in the analysis. Results suggest that a fixed formula in which days 8-19 of the menstrual cycle are considered to be the fertile window would provide the appropriate basis of a simple, effective, family planning method.
