Women can use calendar calculations to identify the fertile time. These calculations were developed in the 1930s to estimate the days the woman is fertile in the current cycle based upon her past cycle lengths. The calculations take into account the lifespan of the egg and sperm and allow for variation in the timing of ovulation from one cycle to the next. If a woman uses a calendar calculation, the interval of time identified as potentially fertile is also longer than the actual time she is fertile. The more a woman's cycle length varies, the longer is the interval identified as potentially fertile.
Recent research shows that the actual fertile time lasts for only about six days each cycle.
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