PIP: Experience with family planning mehods requiring periodic sexual abstinence has been varied. During the last decade interest has centered on 2 methods, the cervical mucus and the sympto-thermal, which are based on identifying the onset of the fertile period. During the 1970s, the Australian physicians John and Evelyn Billings developed the cervical mucus method, in which changes in the quanitity and characteristics of cervical mucus are used to determine the moment of ovulation. The sympto-thermal method depends on identification of the slight rise in basal body temperature that occurs in the latter part of the menstrual cycle as well as cervical mucus changes and sometimes the calendar to determine the fertile period. The Catholic Church has been the main proponent of periodic abstinence methods, but since 1973 the World Health Organization has invested US$3.3 million on research in such methods. The Billings method requires differentiating between dry, wet, and very wet mucus in the vagina and between different consistencies of mucus. The method ususally requires 1-3 months for instruction and sexual abstinence is usually recommended for the 1st month. The average number of days of required abstinence was 9 in a study of 66 women and 15-18 in a study of 870 women. Many women with short menstrual cycles do not experience postmenstrual dry days, in which case abstinence may be required as many as 13 days out of 23. 18.8% of users of periodic abstinence methods in 1 North American study became pregnant in the 1st year, but most were using the calendar method. Women who desired to terminate childbearing had only about 1/2 as many failures with periodic abstinence methods as did women wishing to postpone a birth.^ieng