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In: Jeffcoate SL, ed. Ovulation: methods for its prediction and detection. Chichester, England, John Wiley, 1983. 33-47. (Current Topics in Reproductive Endocrinology Volume 3)This chapter reviews certain recognizable biological effects that occur due to the major changes in the circulating blood levels of estrogen and progesterone and discusses the ongoing use of these biological signals for the self detection of ovulation and the fertile phase of the cycle. These biological changes include the basal body temperature, changes in the cervix and its mucus secretion, mittelschmerz, and the menstrual cycle molimina. The calculation or calendar method is the oldest technique for determining the fertile period and followed the work of Ogino (1930) and Knaus (1933). The fertile phase of the cycle was identified from the records of the previous 6-12 menstrual cycles. The potential fertile period was then calculated on the following basis: define the shortest and the longest menstrual cycle over the preceding 6 and preferable 12 cycles; the 1st day of the potentially fertile phase is the longest cycle minus 11 days. For a women whose menstrual cycles have varied between 26-31 days, the potential fertile period would be days 8-20 of the cycle. The greatest weakness of the calendar calculation is that it depends on a prediction, based on the menstrual history, of what is likely to occur and not on what is actually taking place. Very rapid electronic thermometers are now available which offer considerable advantages over the clinical thermometer. The daily taking and charting of the basal body temperature (BBT) is the simplest and most widely used method for detecting ovulation. To overcome the drawbacks of the calendar method and the BBT method for identifying the fertile period, John and Evelyn Billings of Melbourne in the early 1970s developed the ovulation method. Self recognition of cervical mucus symptoms provides the woman with a simple means of detecting the fertile phase of her cycle and the likely time of ovulation. Individual cycle variation in the preovulatory duration of the symptoms limits the position of the prediction, yet the "peak" day correlates better with the time of ovulation than the shift in BBT. In addition to effects on cervical mucus, estrogen also changes the morphology of the cervix. The preovulatory rise in estrogensoftens the tissues of the cervix and opens the cervical os. The softened cervix and gaping os with a cascade of clear mucus is a sign of optimal estrogen response and of imminent ovulation. A World Health Organization (WHO) multicenter study of the ovulation method provided a substantial amount of information of the normal menstrual cycle of a large number of women of proven fertility in the age group 18-39 years whose cycles were not influenced by the use of hormonal or other contraceptive methods.