Past practices of Natural Family Planning (NFP) have included such techniques as: 1) calendar rhythm in which a constant mathematical relationship was calculated between the day of ovulation and the beginning of the succeeding menstrual period, 2) temperature rhythm in which sexual intercourse is permitted by observing that the basal body temperature rises at the time of ovulation, 3) a combination of calendar and temperature rhythm methods, 4) paper test strips which measure glucose and electrolytes in vaginal secretions, 5) electronic devices of various sorts which record changes in the potentials of the pelvic organs at the time of ovulation, and 6) recent developments in computer technology which measure a number of variables. All of these techniques depend upon the identification of the fertile and infertile periods of the menstrual cycle. More recent studies by the World Health Organization have analyzed the teaching phase of NFP and the effectiveness phase of NFP using the detection of ovulation by following changes in the quality of cervical mucus. These studies tested 869 women with varied backgrounds in 5 countries. The data support the fact that many motivated women can detect ovulation reasonably accurately by following changes in their cervical mucus (the Billings method), although teaching time and motivation are considerable. Long-term and detailed data are necessary to determine the value of these methods. The advantage of NFP is the avoidance of drugs and devices. Disadvantages are that demands are placed on the sex life of couples, and there are associations between failures and increased rates of congenital abnormalities.