安全期避孕的缺陷
安全期避孕的三种方法（基础体温法、日期计算法和宫颈黏液法）都需要专家的专业指导，不应该自行去盲目尝试。然而，甚至合并运用和在医生的帮助下运用，它们也不太可靠。安全期避孕对那些最长月经周期与最短月经周期相差10天以上的妇女就根本不适用。生理不适和情感波动可能改变月经周期的时程，况且某些女子在性交后发生反射性的排卵也不是不可能的。安全期避孕除了它的可靠性缺陷以外，它还有使性交的安排屈从于日历和体温计的缺陷。不过，许多人因宗教信仰的原因使用安全期避孕，因为，目前这是天主教惟一认可的避孕方法。

Contraception

Methods of Contraception: Contraceptive Methods for Women：“Rhythm”

Disadvantages
Each of these three versions of the rhythm method (basal temperature, calendar, and Billings method) requires expert professional guidance. No woman should try them on her own. However, even in combination and with the help of a doctor they are not very reliable. The rhythm method does not work at all for women whose longest cycle differs more than 10 days from the shortest. Physical and emotional problems can change the length of cycles, and there is also the possibility that some women may ovulate in response to coitus itself. Apart from its lack of reliability, the method also has the disadvantage of subjecting coitus to the rule of calendar and thermometer. Nevertheless, many people use the rhythm method for religious reasons, since at present it is the only method of contraception approved by the Catholic church.