Anovulatory cycle
While the normal menstrual cycle in the human typically lasts 4 weeks (28 days, range 24–35 days) and consists of a follicular phase, ovulation, and a luteal phase, followed by either menstruation or pregnancy, the anovulatory cycle has cycle
lengths of varying degrees. In many circumstances, menstrual intervals are prolonged exceeding 35 days leading to oligomenorrhea (cycle >35 – 180 days interval), or even longer, amenorrhea. In other cases, menstruation may be fairly regular (eumenorrhea), or more frequent (intervals < 21 days), or there may be a loss of menstrual pattern (menorrhagia, dysfunctional uterine bleeding).
Fertility awareness
Calendar rules may set a standard number of days, specifying that (depending on a woman's past cycle lengths) the first three to six days of each menstrual cycle are considered infertile. Or, a calendar rule may require calculation, for example holding that the length of the pre-ovulatory infertile phase is equal to the length of a woman's shortest cycle minus 21 days. Rather than being tied to cycle length, a calendar rule may be determined from the cycle day on which a woman observes a thermal shift. One system has the length of the pre-ovulatory infertile phase equal to a woman's earliest historical day of temperature rise minus seven days.