Fertility awareness-based (FAB) methods for preventing pregnancy are based upon the physiological changes during the menstrual cycle and the functional lifespan of sperm and ova. FAB methods rely on the following :
- The periodicity of fertility and infertility
- A single ovulation each cycle
- The limited duration of viability of the ovum, which can only be fertilized 12 to 24 hours after release
- The limited duration of viability of sperm (three to five days in cervical mucus and the upper genital tract)
- A woman's ability to monitor cycle length and/or cycle-related symptoms and signs.
These methods, also called "natural family planning," involve identifying the fertile days of the menstrual cycle using a combination of cycle length and physical manifestations of ovulation (change in cervical secretions, basal body temperature) and then avoiding sexual intercourse or using barrier methods on those days. These methods can also be used in reverse to improve the chance of conception by couples attempting to conceive.
Surveys and behavioral research worldwide indicate that women who choose FAB methods do so because of their concerns about side effects and health consequences of other contraceptive methods. Religious constraints appear to play a small part in their method choice . Although many women have concerns about non-FAB methods of pregnancy prevention, most of these women have not embraced FAB methods. Only about 1 percent of women in the United States use FAB contraception  and 3.6 percent use it worldwide . Possible explanations include lack of information, inconvenience, and concerns about efficacy (high typical user failure rate).
- Lack of information — The lack of readily available patient information on FAB methods is likely related to at least three factors: (1) the absence of a profitable product to advertise and sell, (2) provider concern that these methods are not as effective as other choices, and (3) the amount of time needed to counsel patients [4-6]. Studies including both physicians and nurse midwives have shown that relatively few providers routinely include information about FAB methods in their family planning discussions with patients. As an example, a survey of approximately 500 physicians in the United States found that one-third did not mention FAB methods to their patients at all, while 40 percent mentioned them only to selected women . When asked by a patient for information about a FAB method, most physicians described either calendar rhythm or basal body temperature, which are among the least effective FAB methods (see 'Historical methods' below). In another study, nurse-midwives offered little information about FAB methods based on their perception that these methods were not effective or were inappropriate for their patients .
- Convenience — FAB methods can be labor-intensive for the clinician because of the time required for choosing the most appropriate method for an individual woman and for teaching her the method. These methods also require ongoing effort on the woman's part and impact spontaneity in sexual relations. Development of FAB methods that are relatively easy to incorporate into regular clinical services and use may increase interest and availability.
- Efficacy — Typical user failure rates are several-fold higher than correct use failure rates (table 1), similar to other user dependent methods of pregnancy prevention.