安全期避孕
“安全期避孕”只不過就是在月經週期能受精的日期裏禁絕性交，以避免懷孕。儘管這種方法似乎較容易和在原理上說來也有效，但是要掌握這種方法卻是非常複雜和運用起來不大可靠。首要的難題是：確定受精的確切天數或需要禁絕性交的“不安全”時期。一名婦女的輸卵管內只要存在著卵子，她就有懷孕的可能。這就意味著她的受精或“不安全”時期是排卵前、排卵時和排卵後的短暫數日。排卵前禁絕性交是必要的，因為精子在女子體內能夠存活一段時間；排卵時和排卵後禁絕性交也是必要的，因為卵子在排卵後的一定時間內有受精能力。既然月經週期常常是不規則的，所以不能精確預測排卵的日期，只好騰出至少1周來禁絕性交，這恰好是卵子可能受精的時間。考慮到精子可能的存活時間（在輸卵管內存活達5天），還要加上給安全期留有餘地的1天或2天，所以，一般看來，每一個月經週期的受精時期或“不安全的”日期合計達到了約2周。
下一個難題是：什麼時候準確地啟動這兩周的時間？如上所述，每一項因素依據排卵的時刻而定（假如有可能精確地確定這個時刻，“不安全的”日期將會比2周大為縮短）。通常在月經週期的中期會有排卵，或者說得更準確一點，約在下一個月經週期之前的2周排卵。並不走運的是，要預測下一個月經週期的啟動時間是困難的。許多婦女的月經週期沒有規律，尤其年輕女子和近絕經期的婦女尤甚；甚至於月經週期規則的婦女，行經期在2-5天內變動也是完全正常的。
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Contraception |
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Methods
of Contraception: Contraceptive
Methods for Women
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“Rhythm”
The “rhythm method” of contraception consists simply of abstinence from coitus during a woman's fertile days. Although this method seems easy and effective in principle, it is quite complicated and unreliable in application.
The first problem is to determine the exact number of fertile or "unsafe" days that require coital abstinence. A woman can become pregnant only as long as an egg is in one of her Fallopian tubes. This means that her
fertile or "unsafe" days are those shortly before, during, and shortly after ovulation. Abstinence before ovulation is necessary because sperm cells can stay alive inside a woman's body for some time, and abstinence afterwards is necessary because an egg can be fertilized for some time after ovulation. Since menstrual cycles are often irregular and the day of ovulation can therefore not be predicted accurately, one has to allow for a period of at least a week in which an egg could be available for fertilization. To this must be added the possible survival time of sperm cells (up to five days inside the Fallopian tubes), plus another day or two as a margin of safety.
Therefore, on the average, the fertile or "unsafe" days add up to about two weeks of each menstrual cycle.
The next question is: When exactly do these two weeks begin? As mentioned above, everything depends on the moment of ovulation. (If it were possible to calculate this moment accurately, the "unsafe" days would be much less than two weeks.) An ovulation usually occurs in the middle of the menstrual cycle or, to be more precise, about two weeks before the beginning of the next cycle. Unfortunately, it is very difficult to predict the beginning of the next cycle. Many women, especially when they are young, or when they are approaching menopause, have irregular cycles, and even for women with regular cycles a variation of two to five days is quite normal. |
