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日期计算法
第二个推算“不安全的”日期的做法在于保存至少8个月的月经周期记录。利用这种日历记录，挑选出最长月经周期和最短月经周期。然后用最短周期的天数减18，即为现在月经周期“不安全的”第一天；进而用最长周期天数减去11，即为现在月经周期“不安全的”最后一天。举例来说，如果最短周期是25天，那么25-18=7(其现在的月经周期“不安全的”第一天是月经后第7天)；如果其最长周期是31天，则31-11=20(其现在的月经周期“不安全的”最后一天为月经后第20天)。
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最短周期
Shortest cycle
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周期开始后的第一个不安全日
First
unsafe day after beginning of cycle
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最长周期
Longest cycle
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周期开始后的最后一个不安全日
Last
unsafe day after beginning of cycle
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21 days
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-18 =
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3rd day
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21 days
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-11 =
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10th day
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22 days
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-18 =
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4th day
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22 days
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-11 =
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11th day
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23 days
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-18 =
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5th day
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23 days
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-11 =
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12th day
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24 days
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-18 =
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6th day
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24 days
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-11 =
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13th day
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25 days
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-18 =
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7th day
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25 days
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-11 =
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14th day
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26 days
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-18 =
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8th day
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26 days
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-11 =
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15th day
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27 days
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-18 =
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9th day
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27 days
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-11 =
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16th day
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28 days
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-18 =
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10th day
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28 days
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-11 =
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17th day
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29 days
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-18 =
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11th day
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29 days
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-11 =
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18th day
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30 days
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-18 =
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12th day
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30 days
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-11 =
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19th day
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31 days
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-18 =
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13th day
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31 days
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-11 =
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20th day
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32 days
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-18 =
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14th day
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32 days
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-11 =
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21th day
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33 days
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-18 =
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15th day
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33 days
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-11 =
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22th day
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34 days
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-18 =
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16th day
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34 days
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-11 =
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23th day
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35 days
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-18 =
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17th day
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35 days
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-11 =
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24th day
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36 days
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-18 =
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18th day
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36 days
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-11 =
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25th day
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等等/etc.
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等等/etc.
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Contraception
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Methods
of Contraception:
Contraceptive
Methods for Women: “Rhythm”
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Calendar
A second way of finding the "unsafe" days consists in keeping a calendar record of menstrual cycles for at least eight months. Using this
calendar record, a woman selects her longest and her shortest cycle. She then
subtracts 18 days from her shortest cycle to find
the first "unsafe" day of her present cycle. She further
subtracts 11 days from her longest cycle to find
the last "unsafe" day of her present cycle. For example, if her shortest cycle is 25 days, then 25 - 18 = 7 (the seventh day is the first "unsafe" day of her present cycle): If her longest cycle is 31 days, then 31-11= 20 (the twentieth day is the last "unsafe" day of her present cycle).
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