Cycle day of ovulation

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Abstract
The length of the post-ovulatory phase in a composite sample of menstrual cycles increases linearly with cycle length until cycle length reaches about 30 days, but thereafter it ceases to increase (Vollman, 1953; Bailey & Marshall, 1970). By contrast, plotted against the mean cycle length of the individual, the length of the post-ovulatory phase continues to increase roughly linearly with mean cycle length. It is concluded (a) that many long cycles are aberrant occurrences among cycles of normal length and are due to lengthening of the pre-ovulatory phase and (b) that where the mean cycle length is long, ovulation, as in shorter cycles, occurs just before the middle of the cycle, so that the post-ovulatory phase is longer than usual.The most fertile day seems to be the one before mid-cycle. These conclusions are the basis for the following formula for calendar rhythm:If x is the shortest and y the longest cycle in a woman's menstrual record, the ‘unsafe’ days start on Day (½x − 5) and last for (y − x + 8) days.