Natural family planning. II. Basal body temperature and estimated time of ovulation.

Four points on the basal body temperatures (BBT) curve have been correlated with the estimated time of ovulation (ETO), as determined by indirect hormonal parameters, in 74 menstrual cycles from 24 subjects. Only 10 of 66 hormonally normal cycles exhibited a BBT dip (D), and the ETO ranged from D - 2 through D + 3. In 63 of 66 hormonally normal cycles, a BBT nadir (N), first day of BBT rise (F), and BBT coverline endpoint (C) were identified. In these 63 cycles, the ETO ranged from N - 5 through N + 4, F - 6 through F + 3, and C - 5 through C + 4. Biphasic curves were observed in 72 of the cycles (97.3%) and monophasic curves in 2 cycles (2.7%). In at least 5 of 74 cycles (6.8%) the BBT curve gave incorrect information on the ovulatory status of the cycle.