"Rhythm" method or natural family planning -
This method is also called "fertility awareness". It relies on a woman's understanding of her menstrual cycle, in particular, the timing of her ovulation. The basic principle is that a woman avoid intercourse during the time of her ovulation. However the plot thickens. It turns out that sperm can last in a woman's body for up to about four days. Additionally an egg can last in the body for another four days. Assuming that even in the most regularly ovulating patients, ovulation can vary a day or two, all this leads a rather broad window of fertility on either side of ovulation. All things considered, a woman who ovulates on or around day 14 and has a 28 day cycle is going to be likely to conceive from about day 9 to 19.
As a reminder, day one is the first day of the last menstrual period. Length of cycle is the length of time between the first day of one period and the first day of the next period. A woman with a 28 day cycle will ovulate around day 14. A woman with a 30 day cycle will ovulate around Day 16. This is because the length of time between ovulation and the next is always 14 days. Any variation in the length of cycle comes from variation in the first part of the cycle, the follicular phase, the length of time between day one and day of ovulation. Obviously, with this method of birth control, it is very important to know how many days there are between day one and the day of ovulation. Moreover it is important to know this in advance so that the vulnerable window can be plotted on the calendar. If a woman's cycle length varies greatly from cycle to cycle, then the rhythm method is not a very good method for her to use. This is because she can never predict when the ovulation will come. Even if she used ovulation detection test strips, she might still conceive because of the length of time which sperm can last in a woman's reproductive tract. The sperm would be in place well before the ovulation test strip would turn positive. As you might expect this method is for people for whom pregnancy will not be a problem. The efficacy rate for this method hovers somewhere around 75% or better. It is most useful for spacing pregnancies rather than preventing them altogether.