PIP: This review of natural family planning (NFP) focuses on the following: components of the fertile phase; sympto-thermal methods; the history and methodology of NFP (calendar rhythm, basal body temperature, cervical mucus--the Billings Ovulation method); special circumstances--periods of erratic ovulation (puberty, lactation, premenopause, discontinuation of ovulation suppression, cervicitis and vaginitis, ovulation suppression by stress and pharmaceuticals); effectiveness of natural family planning; achieving pregnancy; achieving couple autonomy (confidence in the method, periodic abstinence, dynamics of the learning process, and support systems); problem areas; and delivery systems. The number of users of NFP methods increased from 2.8% of currently married couples in 1973 to 3.4% in 1976. In 1979, 75,000 new clients received training in contemporary NFP, while the number increased to over 100,000 in 1980. NFP is planning for achieving or preventing a pregnancy by the timing of intercourse. A couple can, by observing and recording certain natural symptoms and bodily changes that occur in a woman's menstrual cycle and using the information as a guide, learn to identify fertile and infertile phases in the menstrual cycle. Precise prediction of ovulation forms one of the components of delineation of the fertile phase. Billings pioneered the use of cervical mucus as a single parameter for the prediction of ovulation and its application to NFP. Women are instructed to observe their mucus patterns at the vulva, relying primarily on the sensation of wetness and lubrication, the use of the Kegel exercise, palpation with the finger, a "wipe-through" with toilet paper, or a combination of these observations. In the absence of ovulation, the usual changing mucus pattern is also absent. NFP can be used either to achieve or to avoid pregnancy. When NFP is used to avoid pregnancy, one will encounter method-related pregnancies, teaching-related pregnancies due either to poor teaching or poor learning or both. The major use effectiveness studies are listed in table form, and the results are shown under new headings. To achieve pregnancy, it is the general practice of NFP instructors to teach women to recognize and record their fertility signs and to suggest some months of merely concentrating coitus at the time of maximum fertility. Mastery of NFP calls for both identification of the fertile phase and integration of that knowledge into the couple's sexual decision making and behaviors. Studies are reviewed in terms of the spectre of genetically damaged offspring. NFP instruction is available in nearly every country outside the Soviet bloc.