If your cycles are occasionally irregular, you should not worry too much. However, if your menstrual cycle frequently occurs more often than every 21 days or less often then every 35 days, or if a period is missed altogether, your cycle can be considered irregular.
An occasional irregular cycle or missed period is usually not a sign of a medical problem. However, consistently irregular periods are often an indication that you have irregular ovulation, or are not ovulating at all. If your cycle is irregular, you need to be evaluated by your physician.
'My Calendar' is designed for women with regular cycles. Don't worry about slight variations in your cycle: about 50 percent of healthy women have cycles that are occasionally irregular, and 'My Calendar' takes these irregularities into account. But if your cycle is consistently irregular, using 'My Calendar' may result in misleading information about your fertility. We do not recommend using 'My Calendar' if your cycle is consistently irregular.