There is an old joke which asks the question, "What do you call people who practice natural family planning?" The answer is, "Parents!" This is an amusing anecdote, but also serves as a caveat to this article. There is only one way to completely avoid pregnancy, and that is abstinence. All other methods, including non-natural methods, are not 100% effective. So if a couple simply must not become pregnant, abstinence is the only method that will have the desired outcome. However, that method is not desirable for most couples, for obvious reasons! For many couples, birth control pills, diaphragms, IUD's, or barrier methods are not viable options. Maybe there are religious restrictions, or health concerns. For these couples, natural family planning is the only option which could be considered possible.
The primary tool for avoiding pregnancy naturally is simply a calendar. The woman must be extremely organized and record her monthly cycle on a calendar for a period of three to six months. This will give her a baseline for her body's own menstrual cycle. In general, a woman has a menstrual cycle every twenty-eight days. However, this is simply an average, and does not apply to all women. There can be a range of twenty to forty or more days between cycles, all considered to be normal limits. This is why a woman must be aware of what her normal cycle of days consists of. By recording the first day of her menses on a calendar each month, then counting the days in between, a woman can determine her cycle. The days may vary month by month, but over a three to six month time-frame, you may detect a pattern or calculate an average.
After determining your average cycle, you can now determine your peak fertility days. Peak fertility days are those days on which a woman is most likely to become pregnant if unprotected sex occurs. Generally, a woman's prime fertility occurs just prior to ovulation, at ovulation, and just after ovulation. Ovulation, or the release of an egg from the ovary, generally occurs at the middle point of the menstrual cycle. For instance, if your cycle is consistently twenty-eight days in length, you would probably release an egg on Day 14. Counting back from Day 14 and forward from day 14 for two to three days gives you a window of time in which unprotected sex could result in pregnancy. So, this woman should avoid intercourse from Day 11 to Day 17.
Ovulation cycles may be shorter than this, or eggs may remain active longer than this. It is a very unpredictable method of family planning. It also depends on the organization and self-control of the couple. These factors are why natural family planning very often results in a baby!