安全期避孕
“安全期避孕”只不过就是在月经周期能受精的日期里禁绝性交，以避免怀孕。尽管这种方法似乎较容易和在原理上说来也有效，但是要掌握这种方法却是非常复杂和运用起来不大可靠。

首要的难题是：确定受精的确切天数或需要禁绝性交的“不安全”时期。一名妇女的输卵管内只要存在着卵子，她就有怀孕的可能。这就意味着她的受精或“不安全”时期是排卵前、排卵时和排卵后的短暂数日。排卵前禁绝性交是必要的，因为精子在女子体内能够存活一段时间；排卵时和排卵后禁绝性交也是必要的，因为卵子在排卵后的一定时间内有受精能力。既然月经周期常常是不规则的，所以不能精确预测排卵的日期，只好腾出至少1周来禁绝性交，这恰好是卵子可能受精的时间。考虑到精子可能的存活时间（在输卵管内存活达5天），还要加上给安全期留有余地的1天或2天，所以，一般看来，每一个月经周期的受精时期或“不安全的”日期合计达到了约2周。
下一个难题是：什么时候准确地启动这两周的时间？如上所述，每一项因素依据排卵的时刻而定（假如有可能精确地确定这个时刻，“不安全的”日期将会比2周大为缩短）。通常在月经周期的中期会有排卵，或者说得更准确一点，约在下一个月经周期之前的2周排卵。并不走运的是，要预测下一个月经周期的启动时间是困难的。许多妇女的月经周期没有规律，尤其年轻女子和近绝经期的妇女尤甚；甚至于月经周期规则的妇女，行经期在2-5天内变动也是完全正常的。

Contraception

Methods of Contraception: Contraceptive Methods for Women

“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.