A method and apparatus for predicting the fertility status of a human female subject. The onset of menstruation of the subject is noted. Daily determinations are made of the subjects eccrine sweat pH, beginning not more than 4 days following menstruation. The start of the fertile period is determined as a function of a significant decline in pH, followed by a sharp rise. The peak of this pH rise is indicative of imminent ovulation, which generally coincides with a second pH decline of variable degree. The apparatus provides provisions for automatically taking the pH readings, analysing them, and displaying to the user the daily fertility status.