Note down the shortest menstrual cycle from the last six months, and subtract 18 days from the total number of days of shortest cycle. The number of days that you get after subtracting 18 days, count that number of days from the first day of your last period and mark it as X. X is the first day of the fertile period which is unsafe period.

The following method will help you find out your fertile window. For 8 to 12 months, record the day you start your menstrual period and count the total number of days in that cycle. Note that the first full flow day of your menstrual period is day 1. Then write down the longest and shortest number of days from your monthly tracking.

According to the popular Safe Period Calculator method, days 1 to day 7 are safe and unfertile in a regular menstrual cycle which lasts 26 to 32 days. This considers the premise that the lifespan of the sperm in the reproductive canal is 3 to 5 days. This method calculates that ovulation occurs on the 19 th day.