A time-dependent model of the probability of sexual intercourse, where the probability increases as the time from last intercourse increases, can produce waves in the expected frequency of sexual intercourse on the various dates of the menstrual cycle. These waves occur when the time-dependent model is linked with a period of abstinence, such as is usually found in most societies during the time of the menses. Previously, such waves have been attributed to cyclic hormonal changes in women without considering the full implication of a time-dependent model of sexual intercourse.