Using a scientific method to measure the length of your monthly cycle, Cycles is most accurate when calculating cycles that fall between 26 and 32 days long. This method always marks days 8 through 19 as fertile.
As long as your cycle is within 26 to 32 days, this method will maintain its accuracy regardless of when your ovulation day actually falls.
For shorter or longer cycles, fertility dates are calculated based on your ovulation day. Cycles does this by marking 5 days before and 2 days after your ovulation as fertile. This is because, under optimal conditions, sperm can stay alive inside a woman for up to 5 days. It’s been reported that women can conceive up to 2 days after ovulation.
Please note that your ovulation day may change based on outside factors, such as stress or sickness. Also, Cycles is not intended to use as a means to prevent pregnancy in any way.