The calendar rhythm method helps you predict your most fertile
days each month by knowing your average menstrual-cycle length.
Menstrual-cycle length is the average number of days between the
first day of a menstrual-cycle and the first day of the next
menstrual cycle. Menstrual-cycle length normally varies by a day
or two each month. The average length of the menstrual cycle is
28 days, plus or minus two days. That said, normal menstrual
cycles can be as short as 24 days or as long as 32 days. But
ovulation (release of an egg) almost always happens 14 days
before the first day of menstrual bleeding. You need to know how
to predict what day of your cycle you are most likely to
ovulate. Knowing your fertile time can help with planning or
preventing pregnancy. Your fertile time is when the egg is
released and can be fertilized. Predicting the fertile time is
based on a 28-day menstrual cycle: To determine this apply the
following formula: For example, if your cycles range between 25
and 31 days, then: Your fert