A novel method of determining the onset of the fertile period of a human female is disclosed comprising the step of monitoring the axillary secretions of said female during the course of her menstrual cycle to determine variations in the concentration of androstenol or dehydroepiandrosterone sulfate. A first significant increase in the concentration of either of those compounds following the cessation of menses is indicative of the onset of the fertile period. In each case, a follicular phase peak occurs which exceeds by at least twice the concentration of those compounds present at any other time during the menstrual cycle. As a result, the monitoring of these compounds may act as reliable indicators predicting the occurrence of ovulation in five to seven days, and/or indicating contraceptive measures can still be taken to avoid pregnancy or sexual intercourse can promote fertility.