A statistical approach to the determination of the fertile period.

17 beta-Estradiol (E2), basal body temperature (BBT) and luteinizing hormone (LH), were measured in normally menstruating women. The preovulatory E2 observations were characterized by variations about a constant level during the preovula-peak. The BBT observations typically varied about a constant level during the preovulatory phase then about a higher constant level during the postovulatory phase. The change points in E2 and BBT were detected using a Bayesian detection rule and a midcycle 'fertile period' was marked off. The midcycle 'fertile periods' appear to bracket the ovulation event and the method shows promise as a natural family planning method. Several practical limitations of the method are discussed.