What does the calendar method of birth control require the client to subtract from the shortest cycle?

The calendar method of birth control, also known as the rhythm method, involves tracking a woman's menstrual cycles to predict fertile days. To determine the most fertile period, the shortest cycle length is used as a reference. The correct approach is to identify the shortest cycle and subtract 18 days from that number to find the first fertile day.

This subtraction is based on the understanding that ovulation typically occurs around 14 days before the start of the next menstrual period. The 18-day adjustment accounts for the variability and helps to ensure that the fertile window is accurately captured for individuals with shorter cycles, allowing them to avoid sexual intercourse or use additional contraceptive methods during this time of high fertility. Therefore, subtracting 18 days from the shortest cycle is essential for effectively utilizing the calendar method.