A fixed formula to define the fertile window of the menstrual cycle as thebasis of a simple method of natural family planning

A significant number of women worldwide use periodic abstinence as their me thod of family planning. Many of them use some type of calendar-based appro ach to determine when they should abstain from unprotected intercourse to a void 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 ferti le 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 p regnancy using this formula. Information a bout the variable probability of pregnancy on different cycle days relative to ovulation also was considere d 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 provid e the appropriate basis of a simple, effective, family planning method. CON TRACEPTION 2000;60:357-360 (C) 2000 Elsevier Science Inc. All rights reserv ed.