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 20. 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. In this case, the Temperature Shift Rule is satisfied on Day 19, a day earlier than the Peak Day Rule. But because the fertile phase does not end until BOTH of these rules have been satisfied, the fertile phase ends on Cycle Day 20 once the Peak Day Rule is satisfied.
Learn the rules of the sympto-thermal method of fertility awareness.