Abstract: A micro-computer-based instrument for characterizing a woman's menstrual cycle by measuring her temperature (BBT) each day and storing and analyzing those temperatures. The instrument defines, with user interaction, a time window each day during which it will accept a temperature measurement within a predetermined acceptable temperature range taken with the aid of a thermistor temperature probe than can be placed under the woman's tongue. The analysis carried out on the collected temperature data includes comparing, from the eighth day of a cycle, two average temperatures: M.sub.F (average of a presumed follicular phase) and M.sub.L (average of a presumed luteal phase). Comparison continues until the difference between M.sub.L and M.sub.F exceeds a predetermined level thereby defining a second phase of the cycle indicating that ovulation has occured.