Step 1: Plan on tracking your menstrual cycle for 8 to 12 months.
Step 2: Day 1 will be the first day you start menstruation
Step 3: Pick the longest and shortest of the cycles from your monthly tracking.
Step 4: The first day of your fertility window is determined by subtracting 18 days from the length of your shortest cycle.
If your shortest menstrual cycle was 26 days, subtract 18 from 26, which gives you the number 8, this means that the first day of your fertility window starts on the 8th day of your cycle.
Step 5: The last fertile day is determined by subtracting 11 from the length of your longest cycle. If 32 days was your longest menstrual cycle, take 32 and subtract 11, which give you 21.
This means that the last day of your fertile period is on the 21st day of your cycle .
The time in between these is considered your fertility window. Your fertility window would be from the 8th day to the 21st day of your cycle.
Your ovulation is expected to occur on one day during this time frame. You cannot get pregnant everyday during this time, but it is during one 12-24 hour time frame during this window that pregnancy can occur.