Determining the Fertile Window using the Sympto-thermal Method of Fertility Awareness
This person's fertile phase begins on Cycle Day 9 when cervical fluid production begins. Since this person experienced a SUDDEN temperature shift on Day 17, the Peak Day Rule is satisfied on the 3rd day after their Peak Day, which in this case occurs on Cycle Day 18. The Temperature Shift Rule is satisfied on the 3rd consecutive day of a basal body temperature that is above the coverline, since that third temperature is at least 0.3 °F (0.15 °C) above the coverline. Because the fertile phase does not end until BOTH of these rules have been satisfied, the fertile phase ends on Cycle Day 19 once the Temperature Shift Rule is satisfied — even though the Peak Day Rule was satisfied on the previous day.
Learn the rules of the sympto-thermal method of fertility awareness.