The relationship of the post-ovulatory phase of the menstrual cycle to total cycle length

The basal body temperature was recorded by 1353 healthy fertile women aged 18 to 49 years inclusive through 12,247 cycles. The post-ovulatory (hyperthermic) phase of the cycle was significantly longer by 1·31 days in cycles with a slow or staircase rise of temperature than in cycles with an acute rise. The hyperthermic phase increased in length in a rectilinear fashion from 10 to 13 days as the total cycle length rose from 22 to 29 days; over total cycle lengths from 29 to 33 days the length of the hyperthermic phase remained around 13 days.